Found this when this happened to me (on a multiplayer file w/1 friend) and because I found something that worked for me, I figured I'd say in case it helps anyone else.
When it crashed every time we tried to go back into the lower city, this is what we had done: - Went to kill Gortash without having taken out the steelwatch. - We may have left some agro in the area (tried both fighting our way out and sneaking out, both options wouldn't let us go back). - We convinced Gortash we were buddies and he said he would meet us somewhere, we then turned around and stabbed him in the back out of dialogue.
When it worked, this is what we did: - We reloaded to a save before we left the lower city. - We took out the steelwatch/did the ironthrone. - We actively picked an 'attack' option in the dialogue. - We made sure not a single enemy was alive when we tried to leave the area.
Then it worked. We kept the same party, still had loads of unfinished quests etc - these things didnt seem to matter. Not sure it would help anyone else, but just in case.. as it was quite frustrating to feel locked out so close to the end of that playthough.
