One of my DO servers that we are using as a test environment works well most of the time, but at seemingly random times, the server responds with a “502 Bad Gateway” error.

In the nginx logs, I can see the following error message:
2019/06/27 13:46:15 [error] 1013#1013: *19539 connect() to unix:/var/run/php/php7.3-fpm.sock failed (111: Connection refused) while connecting to upstream, client: 117.204.xxx.xxx, server: subdomain1.sometestdomain123.xyz, request: “POST /client/communicationNotifyAlerts HTTP/2.0”, upstream: “fastcgi://unix:/var/run/php/php7.3-fpm.sock:”, host: “subdomain1.sometestdomain123.xyz”, referrer: “https://subdomain1.sometestdomain123.xyz/client/api1”

I’m guessing this could be happening after updating our php code but not performing a restart? Could this be caused by something else.