Server keeps crashing :(

Here's whats popping up in the crash reports:

Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkFoomodification(rCArrayList.java:819)
at java.util.ArrayList$Itr.next(ArrayList.java:791)
at net.minecraft.server.WorldServer.getTileEntities(WorldServer.java:546)
at net.minecraft.server.EntityPlayer.h_(EntityPlayer.java:165)
at net.minecraft.server.World.entityJoinedWorld(World.java:1245)
at net.minecraft.server.WorldServer.entityJoinedWorld(WorldServer.java:511)
at net.minecraft.server.World.playerJoinedWorld(World.java:1227)
at net.minecraft.server.World.tickEntities(World.java:1125)
at net.minecraft.server.WorldServer.tickEntities(WorldServer.java:428)
at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:563)
at net.minecraft.server.DedicatedServer.q(DedicatedServer.java:213)
at net.minecraft.server.MinecraftServer.p(MinecraftServer.java:473)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:405)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)

any help would be GREATLY appreciated