Weird bad gateway error on WordPress.

The site works fine but sometimes when I try to post a new post or reply to a comment the backend keeps loading until timeout and then the whole backend gives a bad gateway error but the site frontend homepage is working and it also gives a bad gateway error if you select a post, only the homepage keeps working.
I have to reboot the server for everything get back to normal and it happens every 3-4 inputs(posts or comments).
My specs:
2gb ram
3 cores cpu
Centos 7
Nginx and php-fpm 8.
The problem started to happen (I think) after switching from suphp and apache to php-fpm and nginx.

Hi, check the website error log at the problem moment. Maybe the website unable to save the post and you have the timeout error.
Also, 2GB RAM is too low in 2021 and you must not use ClamAV and many other things.