Invalid Server crash

I am getting this error, and then the server crashes (Kinda)
I am getting "Read timed out" and players cant join.
But for some kind of wierd reason, it still displays when somone is using "/shout <message>" (From the LilyShout plugin)

17:02:38 CONSOLE: at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
17:02:38 CONSOLE: at java.util.HashMap$KeyIterator.next(Unknown Source)
17:02:38 CONSOLE: at net.minecraft.server.v1_6_R3.EntityTracker.updatePlayers(EntityTracker.java:149)
17:02:38 CONSOLE: at net.minecraft.server.v1_6_R3.MinecraftServer.t(MinecraftServer.java:583)
17:02:38 CONSOLE: at net.minecraft.server.v1_6_R3.DedicatedServer.t(DedicatedServer.java:227)
17:02:38 CONSOLE: at net.minecraft.server.v1_6_R3.MinecraftServer.s(MinecraftServer.java:488)
17:02:38 CONSOLE: at net.minecraft.server.v1_6_R3.MinecraftServer.run(MinecraftServer.java:421)
17:02:38 CONSOLE: at net.minecraft.server.v1_6_R3.ThreadServerApplication.run(SourceFile:583)
17:02:38 CONSOLE: Encountered an unexpected exception ConcurrentModificationException

What do you mean by it kinda crashes? When do you get the error? And which version of lily pad are you using?

Ehm yea.. it does not crash, like it closes, it just crashes and no one are able to join
I only get the "Read timed out"
It can not be the lilypad version, cus i am using the same version on all servers, and this server is the only one, that crashes

So it doesn't crash? I'm confused, it closes?

And are you running the 1.7 version of lily pad?