My story of this bug (don't read if you're not interested): I started having this same problem today with NPCs everywhere (except guards, Madesi and Brand-Shei) after I completed the quest in which you have to kill Mercer Frey. As a result, I couldn't even talk to the first guy you meet in the Twilight Sepulcher, thus rendering the follow-up quest uncompleteable. I'm using a mod called Sneak Tools, which allows me to sneak up behind people and slit their throats (and therefore alters the player's interaction with NPCs). Sometimes it doesn't work, though. So I started doing that. Apparently, I could loot their corpses. After some minutes, the throat slit animation started to play randomly as I was wandering outside the Riften Fishery. Then I realized that Sneak Tools was probably what caused the whole bug. It was strange, since Sneak Tools has never tried to catch up on missed throat slit attempts before. What has worked for me: Disable Sneak Tools.Load game.Save in a new slot.Exit game.Re-enable Sneak Tools (optional).Load new save.If you're not using Sneak Tools, then I have no idea what could have caused your bug. I still posted this here because there might be other people using Sneak Tools who have this problem.