It doesn’t seem to matter, before the category IDs there was a string with category slug and the result was the same.
For category 3, 12 WordPress loads the right single.php, the same for category 6 and 11.
And if I only make a new post in category 15, it will take me to single-cat2.php, but not if I choose to put the post in both category 1 and 15, somehow WordPress loads the posts from category 15, but on the way from category to single, WordPress loads single-cat1.php, instead of single-cat2.php as it should.
So the code works, but not if one post are put in two different categories. :/
