Happened two times on our server.
2 People playing, one just created the nether (first time) or
explorers the overworld (second time).

A lot of these are thrown until you stop the server with ^C

java.util.ConcurrentModificationException
at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1115)
at java.util.TreeMap$KeyIterator.next(TreeMap.java:1169)
at qu.a(SourceFile:574)
at qu.a(SourceFile:554)
at bfw.a(SourceFile:259)
at bfw.a(SourceFile:114)
at qt.b(SourceFile:160)
at qt.a(SourceFile:204)
at qu.a(SourceFile:766)
at net.minecraft.server.MinecraftServer.a(SourceFile:361)
at net.minecraft.server.MinecraftServer.s(SourceFile:384)
at pf.run(SourceFile:716)