[SEVERE] Unexpected exception net.minecraft.server.MinecraftException

Hey,
I dont know why and when i get this, nor if it changes something, but i get this error in my console and wanted to post it so you can probably tell me what it is:

Code:

2011-06-01 15:14:04 [SEVERE] net.minecraft.server.MinecraftException: Failed to check session lock, aborting
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.PlayerNBTManager.b(SourceFile:76)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.World.k(World.java:2092)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.World.w(World.java:155)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.World.save(World.java:145)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.World.doTick(World.java:1569)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:391)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:311)
2011-06-01 15:14:04 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-06-01 15:14:04 [SEVERE] Unexpected exception
net.minecraft.server.MinecraftException: Failed to check session lock, aborting
    at net.minecraft.server.PlayerNBTManager.b(SourceFile:76)
    at net.minecraft.server.World.k(World.java:2092)
    at net.minecraft.server.World.w(World.java:155)
    at net.minecraft.server.World.save(World.java:145)
    at net.minecraft.server.World.doTick(World.java:1569)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:391)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:311)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)

Informations about the Rootserver:

Code:

1.+2.: Linux zulu1389.server4you.de 2.6.32-5-amd64 #1 SMP Mon Mar 7 21:35:22 UTC 2011 x86_64 GNU/Linux

3.java version "1.6.0_18"
OpenJDK Runtime Environment (IcedTea6 1.8.7) (6b18-1.8.7-2~squeeze1)
OpenJDK 64-Bit Server VM (build 14.0-b16, mixed mode)

4. I don't run any wrapper.
5. Running RB:818
6. got a start.sh:
#! /bin/sh
BINDIR="$(dirname "$(readlink -fn "$0")")"
cd "$BINDIR"
java -Xincgc -Xmx10G -jar craftbukkit-0.0.1-SNAPSHOT.jar

7.Plugins:
AuthMe
ChestLock
Essentials + GroupManager
Falsebook
iChat
iConomy + ChestShop
MagicCarpet
mcMMO
ModTRS
MultiVerse
Scavenger
Towny
VanishNoPickup
WorldEdit
WorldGuard

If I would know at which action this error occurs, I would test, but I don't know.. I can't run the server a whole day without plugins, cause there are protections and so on, the whole world would be on chaos^^
Sorry 4 my english

Pls update it to the newest bukkit. I love ur plugin but its kinda bugy in new version . Thx a lot.

Try to..
1.stop server.
2. goto your world directory
3. delete session.lock file (don't forget to backup this file)
4. run server again

i had the same problem with my server and it seems to have been caused by an incompatibility with either the latest bukkit build or minecraft update, i went through all my files to see witch were conflicting and it was a problem with multiverse, you should try it once without that

actually this was happening to me on my server just now after i tried to instal the worldedit and worldguard and rhino whatever