I discovered an amazing bug with which you can move the dragon through End GateWay. But that's not all. If during the return of the dragon, hit it, during the teleportation, it is possible to copy it. The first will be on the island, the second will be behind the island.
One way to fix a bug is to add an event when passing through this block. That it would be possible to catch this moment, and simply to forbid it to do.
I suspect the crash of the server was just due to the copying. Dragons were copied a cycle, and the server crashed from the amount.
Crash log from another report:

https://hub.spigotmc.org/jira/projects/SPIGOT/issues/SPIGOT-3970?filter=allopenissues

(sorry for my english)