I had the same problem with a member of a run using heroes, then he do the questline with the heroes and he could go in the last room.
My friend who was using the heroes system was able to enter with his alt who didn't have the key.
Sorry if I wasn't clear with my initial post. I made it at 3am.
This post has been edited by Eziekieal - July 25, 2016, 15:59:34.