Server crash (java.util.ConcurrentModificationException)

My server is running perfectly on 14w30c, but is not able to run for more than a few seconds when using 14w31a.

Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$KeyIterator.next(Unknown Source)
at qi.a(SourceFile:163)
at net.minecraft.server.MinecraftServer.y(SourceFile:614)
at pk.y(SourceFile:303)
at net.minecraft.server.MinecraftServer.x(SourceFile:530)
at net.minecraft.server.MinecraftServer.run(SourceFile:446)
at java.lang.Thread.run(Unknown Source)