Hello @yakir_yakir,
I think the issue could be in AJAX and permissions. Please check your server error log that will provide you some hint regarding the error.
Does your hosting have any security type software on it that attempts to block rogue requests? It may be worth having a word with them (and sending them the 403'ing url) to see if they can shed any light on it for you.
Or else try reset permissions, probably some files just lost their execution bit:
find . -type f -exec chmod 644 {} \;
find . -type d -exec chmod 755 {} \;
I hope this will help you. If still you face any issue, let me know.
If this helps you, please accept it as solution and give kudos.
Regards.