Bukkit server works for my account. Not for others

I posted this already on the Minecraft Forums and someone told me to come here instead, so I'm just gonna copy and paste the original question. I hope this doesn't count as spam or anything?

I'm a bit of a n00b and only started a server on Friday. I decided to get the CraftBukkit server yesterday and so far so good!

Today I had to go somewhere for most the day, so decided to leave the server on for friends to join. I installed WorldGuard and LightVote so people couldn't grief while I'm not there and so they could change the time of day. I also put iConomy on, but haven't bothered to work it out yet.

I got back and decided to prepare my friends account for when he gets back from holiday. He doesn't really care that I know most his passwords, so I logged on and remembered I hadn't changed the settings so that beds work with LightVote installed, because I wanted to set his spawn point. Without stopping the server, I changed it and then tried again and remembered I had to stop it. Upon starting it up again, I couldn't connect. But on my account that I use at home, that connects perfectly fine. It definitely isn't an IP issue, because it doesn't work with an account signed in on the server.

I thought maybe it was because I'm an OP, so I 'opped' my friend and it still wouldn't work.

Why is this? Remember, I'm a n00b at this stuff so don't act like I should know. I'm on a Mac and using the same computer for the server.

This is what happens on Terminal:

21:07:30 [INFO] Done (0.145s)! For help, type "help" or "?"
21:07:39 [INFO] Disconnecting Sunydee [Internal IP was here]: Internal server error
21:07:39 [INFO] Thread exception
21:07:39 [WARNING] Failed to handle packet: java.lang.NullPointerException
java.lang.NullPointerException
at net.minecraft.server.EntityPlayer.a(EntityPlayer.java:311)
at net.minecraft.server.EntityHuman.a(EntityHuman.java:338)
at net.minecraft.server.Entity.e(Entity.java:941)
at net.minecraft.server.PlayerNBTManager.b(SourceFile:183)
at net.minecraft.server.ServerConfigurationManager.b(ServerConfigurationManager.java:102)
at net.minecraft.server.NetLoginHandler.b(NetLoginHandler.java:85)
at net.minecraft.server.NetLoginHandler.a(NetLoginHandler.java:74)
at net.minecraft.server.Packet1Login.a(SourceFile:43)
at net.minecraft.server.NetworkManager.b(NetworkManager.java:215)
at net.minecraft.server.NetLoginHandler.a(NetLoginHandler.java:40)
at net.minecraft.server.NetworkListenThread.a(SourceFile:91)
at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:396)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:308)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)

When reading the logs when I got back, I noticed it stopped before, but came back on

I'm using build #815, and here's where I downloaded it from - http://ci.bukkit.org/job/dev-CraftBukkit/815/

 

Im sorry, but you shouldnt be using build 815 because it it completely unstable. As you can see, it is an "Internal server error". The Problem is, that Multiplayer in Version 1.6 is totaly buggy and will get fixed next week, according to Notchs Twitter. So youre Problably better off, waiting for a recommended 1.6 Bukkit Release or downgrading to 1.5. I know this doesnt really help solve the Issue, but that is the way it is.

 

Why did it suddenly come to a halt? Obviously, I purposely stopped the server, but now it's stopped completely.

And why is it only my account that works? Not sure if I wrote it in the original question, but I had signed in on my friends account on the same computer and that didn't work.

The website I got it from had a blue ball saying it was a success. It would be red if it wasn't a success.

What's this? Will this work? - http://ci.bukkit.org/job/dev-CraftBukkit/816/ - I'm desperate to play online with friends because we have some time off school.

How can I be notified with updates? Remember, I'm new at this stuff.

 

The new build is full of errors, I suggest you downgrade to 1.5 for now and use build #803