text: I have had this happen twice today. First time I was in a normal game that I created with password, no other characters came in and I never left. I got the quest to pop up to kill Baal's minions, then after I finished the minions the quest button popped up and said could not complete.
  Now, I am in a nightmare game alone with password, got through act 1 and all the way to the canyon and quest button pops up saying cannot complete.
  I am gonna create new game to finish, but just wondering if anyone had any insight as to why this keeps happening.
  Sorry if this should be in bug reports, wasn't sure.