Severe Errors in server console and Internal Server Error. Help please.

I am having quite a few problems with my server. It all started basically when I tried updating it to 1.6.1 After doing so I started experiencing major lag issues with commands and other things of the sort and many server crashes.
Finally I chose to just downgrade back to 1.5.2 to hopefully resolve these issues. I took any plugins that I had already updated when i went to 1.6.1 and changed them back to their 1.5.2 versions, replaced the 1.6.1 jar with the 1.5.2 jar and when I went to join I had gotten the Internal Server Error. Thinking it was just something with my plugins I saved all the configs of everything except for multiverse (Don't want to lose my worlds if possible) deleted the plugins, replaced them all and replaced the configs. Still nothing.
At this point I chose to check my server.log file for anything and found many severe errors. Here are my logs, they are exactly the same every time I start the server. I had to delete a few areas of the log to make sure I could post this, but I made sure to keep the lines around each place where there is an error so you can see what is happening right before the error shows up.

2013-07-03 15:36:26 [INFO] Starting minecraft server version 1.5.2
2013-07-03 15:36:26 [INFO] Loading properties
2013-07-03 15:36:26 [INFO] Default game type: SURVIVAL
2013-07-03 15:36:26 [INFO] Generating keypair
2013-07-03 15:36:27 [INFO] Starting Minecraft server on s3.hosthorde.com:25598
2013-07-03 15:36:27 [INFO] This server is running CraftBukkit version git-Bukkit-1.5.2-R1.0-1-gf46bd58-b2793jnks (MC: 1.5.2) (Implementing API version 1.5.2-R1.1-SNAPSHOT)
2013-07-03 15:36:28 [INFO] ----- Bukkit Auto Updater -----
2013-07-03 15:36:28 [INFO] It appears that you're running a Development Build, when you've specified in bukkit.yml that you prefer to run Recommended Builds.
2013-07-03 15:36:28 [INFO] If you would like to be kept informed about new Development Build releases, it is recommended that you change 'preferred-channel' in your bukkit.yml to 'dev'.
2013-07-03 15:36:28 [INFO] With that set, you will be told whenever a new version is available for download, so that you can always keep up to date and secure with the latest fixes.
2013-07-03 15:36:28 [INFO] If you would like to disable this warning, simply set 'suggest-channels' to false in bukkit.yml.
2013-07-03 15:36:28 [INFO] ----- ------------------- -----
2013-07-03 15:36:28 [INFO] [SignEdit] Loading SignEdit v3.3.2
2013-07-03 15:36:28 [INFO] [GroupManager] Loading GroupManager v2.0 (2.11.1) (Phoenix)
2013-07-03 15:36:28 [INFO] [WorldEdit] Loading WorldEdit v5.5.6
2013-07-03 15:36:28 [INFO] [ChatControl] Loading ChatControl v2.6.0
2013-07-03 15:36:28 [INFO] [BuycraftAnnouncer] Loading BuycraftAnnouncer v2.0
2013-07-03 15:36:28 [INFO] [Buycraft] Loading Buycraft v5.5
2013-07-03 15:36:28 [INFO] [SimpleAnnouncer] Loading SimpleAnnouncer v1.9
2013-07-03 15:36:28 [INFO] [JukeLoop] Loading JukeLoop v0.1.8
2013-07-03 15:36:28 [INFO] [Vault] Loading Vault v1.2.25-b320
2013-07-03 15:36:28 [INFO] [Multiverse-Core] Loading Multiverse-Core v2.4-b527
2013-07-03 15:36:28 [INFO] [CommandWatcher] Loading CommandWatcher v0.5
2013-07-03 15:36:28 [INFO] [SuperSpleef] Loading SuperSpleef vB 0.1.7
2013-07-03 15:36:28 [INFO] [Admin Chat] Loading Admin Chat v1.1
2013-07-03 15:36:28 [INFO] [mcMMO] Loading mcMMO v1.4.06-beta5-b2062
2013-07-03 15:36:28 [INFO] [WorldGuard] Loading WorldGuard v5.7.5
2013-07-03 15:36:28 [INFO] [Essentials] Loading Essentials v2.11.1
2013-07-03 15:36:28 [INFO] [CommandSigns] Loading CommandSigns v1.9.1.1
2013-07-03 15:36:28 [INFO] [EssentialsProtect] Loading EssentialsProtect v2.11.1
2013-07-03 15:36:28 [INFO] [EssentialsSpawn] Loading EssentialsSpawn v2.11.1
2013-07-03 15:36:28 [INFO] [Multiverse-Portals] Loading Multiverse-Portals v2.4-b548
2013-07-03 15:36:28 [INFO] [Multiverse-Inventories] Loading Multiverse-Inventories v2.5-b335
2013-07-03 15:36:28 [INFO] [EssentialsChat] Loading EssentialsChat v2.11.1
2013-07-03 15:36:28 [INFO] [EssentialsAntiBuild] Loading EssentialsAntiBuild v2.11.1
2013-07-03 15:36:28 [INFO] [Factions] Loading Factions v1.6.9.4
2013-07-03 15:36:28 [INFO] [Vault] Enabling Vault v1.2.25-b320
2013-07-03 15:36:28 [INFO] [Vault][Economy] Essentials Economy found: Waiting
2013-07-03 15:36:28 [INFO] [Vault][Permission] GroupManager found: Waiting
2013-07-03 15:36:28 [INFO] [Vault][Permission] SuperPermissions loaded as backup permission system.
2013-07-03 15:36:37 [INFO] [Essentials] Enabling Essentials v2.11.1
2013-07-03 15:36:38 [INFO] Essentials: Using GroupManager based permissions.
2013-07-03 15:36:38 [INFO] [CommandSigns] Enabling CommandSigns v1.9.1.1
2013-07-03 15:36:38 [INFO] [CommandSigns] Successfully loaded 1 signs
2013-07-03 15:36:38 [INFO] [CommandSigns] Plugin metrics enabled! Thank you!
2013-07-03 15:36:38 [INFO] [EssentialsProtect] Enabling EssentialsProtect v2.11.1
2013-07-03 15:36:38 [INFO] [EssentialsSpawn] Enabling EssentialsSpawn v2.11.1
2013-07-03 15:36:38 [INFO] [Multiverse-Portals] Enabling Multiverse-Portals v2.4-b548
2013-07-03 15:36:38 [INFO] [Multiverse-Portals] - Version 2.4-b548 Enabled - By Rigby and fernferret
2013-07-03 15:36:38 [WARNING] [Multiverse-Portals] Failed Parsing World for: Celestial (World Error, World did not exist or was not imported into Multiverse-Core!)
2013-07-03 15:36:38 [WARNING] [Multiverse-Portals] Portal Celestial has an invalid LOCATION!
2013-07-03 15:36:38 [WARNING] [Multiverse-Portals] Portal 'Celestial' not loaded due to invalid location!
2013-07-03 15:36:38 [WARNING] [Multiverse-Portals] Failed Parsing World for: Celestial1 (World Error, World did not exist or was not imported into Multiverse-Core!)
2013-07-03 15:36:38 [WARNING] [Multiverse-Portals] Portal Celestial1 has an invalid LOCATION!
2013-07-03 15:36:38 [WARNING] [Multiverse-Portals] Portal 'Celestial1' not loaded due to invalid location!
2013-07-03 15:36:38 [INFO] [Multiverse-Portals] 8 - Portals(s) loaded
2013-07-03 15:36:38 [INFO] [Multiverse-Inventories] Enabling Multiverse-Inventories v2.5-b335
2013-07-03 15:36:38 [INFO] [Multiverse-Inventories 2.5-b335] enabled.
2013-07-03 15:36:38 [INFO] [EssentialsChat] Enabling EssentialsChat v2.11.1
2013-07-03 15:36:38 [INFO] [EssentialsAntiBuild] Enabling EssentialsAntiBuild v2.11.1
2013-07-03 15:36:38 [SEVERE] §4Unknown item [] in protect.alert.on-placement list.
2013-07-03 15:36:38 [SEVERE] §4Unknown item [] in protect.alert.on-use list.
2013-07-03 15:36:38 [SEVERE] §4Unknown item [] in protect.alert.on-break list.
2013-07-03 15:36:38 [INFO] [Factions] Enabling Factions v1.6.9.4
2013-07-03 15:36:38 [INFO] [Factions v1.6.9.4] === ENABLE START ===
2013-07-03 15:36:38 [INFO] [Factions v1.6.9.4] Loading board from disk
2013-07-03 15:36:38 [INFO] [Factions v1.6.9.4] Found and will integrate chat with EssentialsChat v2.11.1
2013-07-03 15:36:38 [INFO] [Factions v1.6.9.4] Economy integration through Vault plugin successful.
2013-07-03 15:36:38 [INFO] [Factions v1.6.9.4] NOTE: Economy is disabled. You can enable it with the command: f config econEnabled true
2013-07-03 15:36:38 [INFO] [Factions v1.6.9.4] === ENABLE DONE (Took 191ms) ===
2013-07-03 15:36:39 [INFO] Server permissions file permissions.yml is empty, ignoring it
2013-07-03 15:36:39 [INFO] Done (9.153s)! For help, type "help" or "?"
2013-07-03 15:36:39 [INFO] [0;30;22m[[0;36;22mCelestialPVP[0;30;22m][0;30;22m[[0;36;22mWelcome To CelestialPVP[0;30;22m][m
2013-07-03 15:36:39 [INFO] GroupManager - INFO - Bukkit Permissions Updated!
2013-07-03 15:36:39 [SEVERE] [Buycraft] The specified Secret key could not be found.
2013-07-03 15:36:39 [SEVERE] [Buycraft] Type /buycraft for further advice & help.
2013-07-03 15:37:10 [INFO] Disconnecting 74SweatyPandas [/74.76.76.5:59298]: Internal server error
2013-07-03 15:37:10 [WARNING] Failed to handle packet for 74SweatyPandas [/74.76.76.5:59298]: net.minecraft.server.v1_5_R3.ReportedException: Reading NBT data
net.minecraft.server.v1_5_R3.ReportedException: Reading NBT data
at net.minecraft.server.v1_5_R3.Entity.f(Entity.java:1214)
at net.minecraft.server.v1_5_R3.WorldNBTStorage.load(WorldNBTStorage.java:201)
at net.minecraft.server.v1_5_R3.PlayerList.a(PlayerList.java:186)
at net.minecraft.server.v1_5_R3.PlayerList.a(PlayerList.java:67)
at net.minecraft.server.v1_5_R3.PendingConnection.d(PendingConnection.java:129)
at net.minecraft.server.v1_5_R3.PendingConnection.c(PendingConnection.java:44)
at net.minecraft.server.v1_5_R3.DedicatedServerConnectionThread.a(DedicatedServerConnectionThread.java:41)
at net.minecraft.server.v1_5_R3.DedicatedServerConnection.b(SourceFile:29)
at net.minecraft.server.v1_5_R3.MinecraftServer.r(MinecraftServer.java:581)
at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:226)
at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
Caused by: java.lang.ClassCastException: net.minecraft.server.v1_5_R3.NBTTagFloat cannot be cast to net.minecraft.server.v1_5_R3.NBTTagInt
at net.minecraft.server.v1_5_R3.NBTTagCompound.getInt(SourceFile:125)
at net.minecraft.server.v1_5_R3.EntityLiving.a(EntityLiving.java:1180)
at net.minecraft.server.v1_5_R3.EntityHuman.a(EntityHuman.java:562)
at net.minecraft.server.v1_5_R3.EntityPlayer.a(EntityPlayer.java:92)
at net.minecraft.server.v1_5_R3.Entity.f(Entity.java:1153)
... 12 more
2013-07-03 15:37:47 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:38:10 [INFO] /67.43.7.19:39246 lost connection
2013-07-03 15:38:55 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:40:04 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:41:12 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:41:39 [INFO] [0;30;22m[[0;36;22mCelestialPVP[0;30;22m][0;30;22m[[0;36;22mCheck out our website at celestialpvp.webs.com[0;30;22m][m
2013-07-03 15:42:21 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:43:29 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:44:06 [INFO] /67.43.7.19:38640 lost connection
2013-07-03 15:44:37 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:45:46 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:46:34 [INFO] CONSOLE: Enabled level saving..[m
2013-07-03 15:46:34 [INFO] CONSOLE: Forcing save..[m
2013-07-03 15:46:34 [SEVERE] net.minecraft.server.v1_5_R3.ExceptionWorldConflict: Failed to check session lock, aborting
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.WorldNBTStorage.checkSession(WorldNBTStorage.java:75)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.World.F(World.java:2599)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.a(WorldServer.java:785)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.save(WorldServer.java:769)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftWorld.save(CraftWorld.java:649)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.command.defaults.SaveCommand.execute(SaveCommand.java:30)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:189)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchCommand(CraftServer.java:523)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchServerCommand(CraftServer.java:512)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.an(DedicatedServer.java:262)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:227)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
2013-07-03 15:46:34 [SEVERE] net.minecraft.server.v1_5_R3.ExceptionWorldConflict: Failed to check session lock, aborting
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.WorldNBTStorage.checkSession(WorldNBTStorage.java:75)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.World.F(World.java:2599)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.a(WorldServer.java:785)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.save(WorldServer.java:769)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftWorld.save(CraftWorld.java:649)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.command.defaults.SaveCommand.execute(SaveCommand.java:30)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:189)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchCommand(CraftServer.java:523)
2013-07-03 15:46:34 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchServerCommand(CraftServer.java:512)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.an(DedicatedServer.java:262)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:227)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
2013-07-03 15:46:34 [SEVERE]at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
2013-07-03 15:46:34 [INFO] CONSOLE: Save complete.[m
2013-07-03 15:46:39 [INFO] [0;30;22m[[0;36;22mCelestialPVP[0;30;22m][0;30;22m[[0;36;22mBe sure to go to our forums to keep yourself up to date with events[0;30;22m][m
2013-07-03 15:46:54 [INFO] [0;33;22mThere are [0;31;1m0[0;33;22m out of maximum [0;31;1m100[0;33;22m players online.[m
2013-07-03 15:47:14 [INFO] CONSOLE: Forcing save..[m
2013-07-03 15:47:14 [SEVERE] net.minecraft.server.v1_5_R3.ExceptionWorldConflict: Failed to check session lock, aborting
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldNBTStorage.checkSession(WorldNBTStorage.java:75)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.World.F(World.java:2599)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.a(WorldServer.java:785)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.save(WorldServer.java:769)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftWorld.save(CraftWorld.java:649)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.command.defaults.SaveCommand.execute(SaveCommand.java:30)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:189)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchCommand(CraftServer.java:523)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchServerCommand(CraftServer.java:512)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.an(DedicatedServer.java:262)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:227)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
2013-07-03 15:47:14 [SEVERE] net.minecraft.server.v1_5_R3.ExceptionWorldConflict: Failed to check session lock, aborting
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldNBTStorage.checkSession(WorldNBTStorage.java:75)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.World.F(World.java:2599)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.a(WorldServer.java:785)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.save(WorldServer.java:769)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftWorld.save(CraftWorld.java:649)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.command.defaults.SaveCommand.execute(SaveCommand.java:30)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:189)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchCommand(CraftServer.java:523)
2013-07-03 15:47:14 [SEVERE]at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchServerCommand(CraftServer.java:512)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.an(DedicatedServer.java:262)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:227)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
2013-07-03 15:47:14 [INFO] CONSOLE: Save complete.[m
2013-07-03 15:47:14 [INFO] CONSOLE: Stopping the server..[m
2013-07-03 15:47:14 [INFO] Stopping server
2013-07-03 15:47:14 [INFO] [GroupManager] Disabling GroupManager v2.0 (2.11.1) (Phoenix)
2013-07-03 15:47:14 [INFO] GroupManager - INFO - Scheduled Data Saving is disabled!
2013-07-03 15:47:14 [INFO] GroupManager version 2.0 (2.11.1) (Phoenix) is disabled!
2013-07-03 15:47:14 [INFO] [SignEdit] Disabling SignEdit v3.3.2
2013-07-03 15:47:14 [INFO] Saving players
2013-07-03 15:47:14 [WARNING] DSCT: Socket closed
2013-07-03 15:47:14 [INFO] Saving worlds
2013-07-03 15:47:14 [INFO] Closing listening thread
2013-07-03 15:47:14 [INFO] Saving chunks for level 'New'/Overworld
2013-07-03 15:47:14 [INFO] Saving chunks for level 'New_nether'/Nether
2013-07-03 15:47:14 [INFO] Saving chunks for level 'New_the_end'/The End
2013-07-03 15:47:14 [INFO] Saving chunks for level 'celsurv'/Overworld
2013-07-03 15:47:14 [INFO] Saving chunks for level 'celcre'/Overworld
2013-07-03 15:47:14 [INFO] Saving chunks for level 'celgames'/Overworld
2013-07-03 15:47:14 [INFO] Saving chunks for level 'RedStoners'/Overworld
2013-07-03 15:47:14 [INFO] Saving chunks for level 'BioFinalProject'/Overworld
2013-07-03 15:47:14 [SEVERE] net.minecraft.server.v1_5_R3.ExceptionWorldConflict: Failed to check session lock, aborting
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldNBTStorage.checkSession(WorldNBTStorage.java:75)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.World.F(World.java:2599)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.a(WorldServer.java:785)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.WorldServer.save(WorldServer.java:769)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.saveChunks(MinecraftServer.java:318)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.stop(MinecraftServer.java:349)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:441)
2013-07-03 15:47:14 [SEVERE]at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)

Update, I don't see severe errors in the console anymore, it was just because I had extra portals in multiverse after deleting old worlds that I didn't use anymore. But I am still getting an internal server error, any ideas why?

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

If you updated and then downgraded and you started to get those errors, it's because the server has entities on it, which it doesn't know. So it crashes. Your world is now 1.6 you have to stick with it or use a backup taken in 1.5

 

Or I could always just use MCedit and delete all entities in the entire world. Thankyou xD