Living Cities of Vvardenfell, I believe. But, I found that it makes certain quests very difficult if you can't locate the quest-giver when you're done, and certain people that you are supposed to find are not where they are supposed to be. Additionally, if you use the door-locking mod, certain doors will be locked when you need to get in for a quest. It's also possible that the problem was me, not the mod, so give it a whirl and if you encounter similar problems, you can always remove the mod later.