I've disabled all my addons except one that i can't disable because it says that there aren't the file for the disintallation, i tried to create a default style and tried to edit the "forum_list" and it still happen, i also tried to copy/paste a backup of the forum where there wasn't the problem but nothing changed how can i know if i removed all the custom JS?
edit: the 1st time i saw the error was after i've restored the template bacause there were something outdated
This is almost certainly caused by something like mod_security. It's triggering an error and trying to load an error message and failing, which ends up with it falling back to another page. Your host can likely identify the specific rule behind the issue.
Most likely the REAL issue is a security rule violation for whatever setup the server uses (as Mike said, odds are it's mod_security) and you can find the issue by looking at the server log files and finding what triggered it. If you are self-hosted (VPS/dedi) then you will need to investigate that aspect, otherwise let your hosting provider know about it and have them research it and exclude whatever rule (or modify it) that is triggering.
Odds are that's a standard template statement in all XF installs (I found it in forum_list template) and if it was an issue it would be reflected on other sites also.