Since nobody will give me an answer then I will share with you my solution.
Somehow in my case I found a template inside the upload folder: /wp-content/uploads/custom-templates/[THEME-SLUG]-single.php
Because I use a child theme I created there a new file named [CHILD-THEME-SLUG]-single.php with the same content with same content with [THEME-SLUG]-single.php
It seems to fix the problem. Not sure if is the correct solution but it works ??
