That bit where you're supposed to "Find the Release Lever in the Guardian's Skull" doesn't let you pull the lever sometimes, breaking the rest of the quest. Just happened in veteran, PCNA.
It happens if someone who doesn't have the dungeon story quest (that is, they've already completed it on a prior run) pulls the lever first. It's a pain, because it's so far into the dungeon, and not everyone knows about it or realises that they're running with someone trying to complete the story.
What should happen is that when the lever is pulled, the game should loop through every player in the group and check if they have the story mission active, rather than just checking for the one who activated it.