I am running a php-fpm application behind nginx, and sometimes I am getting 502 responses with the message "upstream sent too big header while reading response header from upstream". I have read lots of posts saying that the solution is to increase the fastcgi_buffers configuration, however I have increased this from fastcgi_buffers 16 64k; all the way to fastcgi_buffers 8 256k; and I still see the issue.

What I would like to know is, how can I try and debug and figure out what exactly is causing the issues. I have access to the nginx access and error logs, but I am not sure what to look for to try and find out why the specific endpoints in question are causing this issue.