Server Crash java.util.ConcurrentModificationException

My snapshot server crashed twice, since last update. Here is error:
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$KeyIterator.next(HashMap.java:828)
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(Thread.java:662)

And crash reports in attachments