My current bugged mission is probably a similar problem that I was having with the Companion quest: I cleared out the dungeon prior to receiving the quest. I have to release one of the Gray-Manes from Thalmor custody, but I see that I've cleared the dungeon on my map and whenever I enter the building it crashes my game to desktop. The companion quest is a common issue: doing one of the quests that frequently gets assigned to that dungeon (helm of winterhold) will prevent the quest from getting assigned by Vilkas. It also won't get assigned if that other quest is active. Theres a console fix or you can wait 30 in-game days for the dungeon to reset. I'm guessing quite a few quests (especially radiant AI quests) could get messed up by having them assigned to locations you've already cleared.