I can change the quest back to anything in Act III but cannot progress to Act IV. My monk's quest is currently set to Act IV: Tyrael at the Gates, but changing it to Begin Quest doesn't help either, it simply crashes the game completely. This only seems to apply to my monk, my other characters work fine. For now.
Help. I really, really don't want to start over with a new character.
Edit: Welp, it's happening for any character entering Act IV now. Yay!