Server crashing issue

So we have the mod on and we made a dungeon. It's a big one with lot's of the custom mobs and bosses but when it comes to the boss mobs (when the chunk they in is loaded up) It crashes the whole server. We have not found out why this happens but we could use some help!

Here is the error log issue:
java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429) at java.util.HashMap$KeyIterator.next(HashMap.java:1453) at...

Server crashing issue