NGINX | recv () failed (104: Connection reset from peer) while reading response header from upstream

I get this error message and tried almost everything I found on the internet that says to fix this problem, but I didn't have sucess :(

recv () failed (104: Connection reset by peer) while reading response header up

I am using WordPress on a VPS, with 8-10 domains in the same WordPress installation. For all other sites, I had no problem, but for the WordPress master site, when I get any page, I get 502 Bad Gateway

None of my pages are working. From what I see, whenever any page on the site is opened, PHP-FPM crashes with below error:

child 991195 exited signal 11 (SIGSEGV) after 18.490300 seconds from start

and this thing keeps happening every time a page is opened on this site.

Please help me with a complete proof of how to determine the root cause with this problem and how to fix it.

Many thanks for your help.