Yes, that foggy looking area looks like it may be inadequate fixing to me as well. Does it have a white-ish, milky look to it, especially when it was wet? If so, it's a fixing problem, either your fix is spent, contaminated, or you just didn't give it enough time. Another round of fixing should take care of it.
Note that if that area were due to an exposure issue, it would likey be less dense, but but since it looks light in a positive, it has more density.
Other than that, since there is so much shadow area, it will be a thin looking negative no matter what The area away from the cloudy part looks ok, given the conditions you've described.