"Failed to check session lock, aborting"

My server has been disappearing from client multiplayer server lists despite being online for connected players.


This is what the disconnected players see despite already connected players being on the server and playing. This error is followed by this repeating unit of spam in the console.

2013-04-14 08:12:50 [SEVERE] net.minecraft.server.v1_5_R2.ExceptionWorldConflict: Failed to check session lock, aborting
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.WorldNBTStorage.checkSession(WorldNBTStorage.java:75)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.World.E(World.java:2714)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.ChunkRegionLoader.a(ChunkRegionLoader.java:118)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.ChunkProviderServer.saveChunk(ChunkProviderServer.java:209)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.ChunkProviderServer.unloadChunks(ChunkProviderServer.java:299)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.WorldServer.doTick(WorldServer.java:196)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:553)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:229)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:471)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:403)
2013-04-14 08:12:50 [SEVERE] at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
The complete error log is here.
http://paste.ubuntu.com/5708623/

I have already tried deleting the session lock file in my main world folder multiple times, but this error pops up every few hours, and the server has to be restarted to allow players to connect again.

 

Try deleting it with the server stopped?