Upstream prematurely closed connection while reading response header

Well, no replies here, maybe my query was too vague or too similar to others. I tried rolling back through older versions back to 19 and they all fail to work. It’s not clear what’s wrong but as noted other apps (wordpress, cacti) that use the same infrastructure (nginx, php-fpm) still work and never stopped. Is there a more detailed debugging step I can put in place?