Player entered the end, attempted to battle the dragon, didn't manage to kill it however it just disappeared after making a pass, error in the server console;
[Server thread/ERROR]: An Entity HardcoreEnderExpansion.Dragon(chylex.hee.dragon.EntityDragon) has thrown an exception during loading, its state cannot be restored. Report this to the mod author

Openeye recorded the issue too in detail: Http://openeye.openmods.info/crashes/cf207884cd241c4bbbe0948da3247be5

Note: Server jar: Cauldron-1.7.2-1.1112.04.1616
If this is a Cauldron error I'll go pop it along to Cauldron and see if they can fix it.

Outdated version of HEE, enable update notifications.