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:, 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: “”

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.

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It appears that whatever the variable p represents is sometimes not properly set. I would recommend reviewing your php-fpm logs in /var/log/php-fpm/ as there may be more detail shown there to help you to track down the issue.