Fun fact about backtracking here (at least according to my experience when testing):
If you loop through the cells from up to bottom and then from left to right, the last test case is way longer to solve!
This order took less than 2 seconds.
.
This order took more than 80 seconds!
.
(And I found the same solution in both cases, obviously.)
For what I know, it could be your problem @MixWit (if you haven’t solved it since your last post).