FastCGI error while reading response header from Upstream
Ubuntu LEMP on 14.04
About 2-1/2 weeks ago we started receiving this intermittent error in our error logs:
2018/01/23 16:38:14 [error] 7003#0: *585202 FastCGI sent in stderr: "PHP message: PHP Notice: Undefined index: p in /usr/share/nginx/html/public/make-pdf/generate-graphs.php on line 5" while reading response header from upstream, client: 184.108.40.206, server: localhost, request: "GET /make-pdf/generate-graphs.php?s=triangle&c=b5 HTTP/1.0", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "220.127.116.11"
The error started occurring randomly as no code updates have been applied in over 2 months - not sure what may have changed for this problem to begin occurring but any help troubleshooting would be appreciated.
I have tried restarting php5-fpm and also restarting nginx but they do not seem to help the problem.