A check showed that the problem was that the quality search boundary was too high. Apparently, the developers decided that there are no excellent materials on low-level locations and trying to find something including them fails the search as a whole.
If you lower the search terms to "2++ or lower (instead of 4++++ or lower), the search runs fine as before.
If you lower the search terms to "2++ or lower (instead of 4++++ or lower), the search runs fine as before.