Repeated server crashes, same error each time

My server keeps crashing every few hours with this error. Any idea what could be causing it?

at net.minecraft.server.PlayerManager.flush(PlayerManager.java:35)
2012-07-10 04:46:50 [SEVERE] at net.minecraft.server.ServerConfigurationManager.tick(ServerConfigurationManager.java:414)
2012-07-10 04:46:50 [SEVERE] at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:568)
2012-07-10 04:46:50 [SEVERE] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:459)
2012-07-10 04:46:50 [SEVERE] at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
2012-07-10 04:46:50 [SEVERE] Unexpected exception

 

Plugins?

 

WordReplace, MultiWorld, WorldEdit, BukkitCompat, ChatCo, CommandBook, ChatColors, ClearInv, Vault, PermissionsEx, Stargate, BanHammer, LogBlock, LWC, Gmlist, WorldGuard, SimpleWarnings, dynmap, BorderGuard, PhysicalShop, ServerEvents, SimplyVanish, Tetris, SimpleJail, EndReset, SimpleWarps, MidiBanks, BlockControl, Time Maintain, TeleportSuite, ColorMe, SimpleSpleef, Torture, MonsterBox, Herochat, mcMMO, Lift, MobRider, MobCatcher, XrayInformer, ServerSigns, CreativeControl, BlockHat, HomeSpawnPlus​