big problem.. bukkit error crashing server often

i dont know what this error means or what its for so could someone please help me figure it out cuz my server is starting to crash alot from this



2011-07-17 10:40:17 [SEVERE] java.util.ConcurrentModificationException
2011-07-17 10:40:17 [SEVERE] at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
2011-07-17 10:40:17 [SEVERE] at java.util.AbstractList$Itr.next(AbstractList.java:343)
2011-07-17 10:40:17 [SEVERE] at java.util.AbstractCollection.removeAll(AbstractCollection.java:336)
2011-07-17 10:40:17 [SEVERE] at net.minecraft.server.World.cleanUp(World.java:1072)
2011-07-17 10:40:17 [SEVERE] at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:447)
2011-07-17 10:40:17 [SEVERE] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:361)
2011-07-17 10:40:17 [SEVERE] at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-07-17 10:40:17 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at java.util.AbstractCollection.removeAll(AbstractCollection.java:336)
at net.minecraft.server.World.cleanUp(World.java:1072)
at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:447)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:361)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-07-17 10:41:41 [INFO] Read timed out
2011-07-17 10:41:42 [SEVERE] null
java.io.IOException: Input/output error
at java.io.FileInputStream.read(Native Method)
at jline.Terminal.readCharacter(Terminal.java:99)
at jline.UnixTerminal.readVirtualKey(UnixTerminal.java:128)
at jline.ConsoleReader.readVirtualKey(ConsoleReader.java:1453)
at jline.ConsoleReader.readBinding(ConsoleReader.java:654)
at jline.ConsoleReader.readLine(ConsoleReader.java:494)
at net.minecraft.server.ThreadCommandReader.run(ThreadCommandReader.java:23)

 

I'm getting something like that too now.

Code:

2011-07-17 13:08:40 [SEVERE] java.util.ConcurrentModificationException
2011-07-17 13:08:40 [SEVERE]     at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
2011-07-17 13:08:40 [SEVERE]     at java.util.AbstractList$Itr.next(Unknown Source)
2011-07-17 13:08:40 [SEVERE]     at java.util.AbstractCollection.removeAll(Unknown Source)
2011-07-17 13:08:40 [SEVERE]     at net.minecraft.server.World.cleanUp(World.java:1070)
2011-07-17 13:08:40 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:447)
2011-07-17 13:08:40 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:361)
2011-07-17 13:08:40 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-07-17 13:08:40 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
    at java.util.AbstractList$Itr.next(Unknown Source)
    at java.util.AbstractCollection.removeAll(Unknown Source)
    at net.minecraft.server.World.cleanUp(World.java:1070)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:447)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:361)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)

I don't know how to fix