My friend and I tried to fight Chronomancer Julra as COOP. When we got to the boss door I could click on the door and proceed, but he did not get a prompt to vote. We ended up having to leave the dungeon. He attempted to leave and teleport back, but his game crashed. (I know this should not have worked as once you leave a dungeon you cannot get back in.)
Details : I started the dungeon with my Key. No error messages appeared.
Thanks
Will