Easy fix here homie. At the beginning of the fortress mission, you deactivate the Infinite Heath cheat and then get hit just once. Once is all you need then reactivate it. I had the same problem and that fixed it. However if it still happens just keep the cheat off until you finish the fortress and all the cut scenes end and you are back out on the map.
Same happened to me. Seems like infinite health is breaking stuff from time to time, re: the previous quest issue mentioned. Very odd. Will keep health off in the future. Hopefully didn't break my game with the last one I tried to do. Really annoying you can't have multiple saves/backups.