I increased log level to debug and below is what I received.

[error] 7901#7901: *47646 upstream sent unexpected FastCGI record: 3 while reading response header from upstream, client: xx.xx.xx.xx, server: www.xxxxx.com, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/php-fpm/xxx.fpm.sock:", host: "www.xxxxx.com"

Is there a way I could troubleshoot this or get more information?