Nginx 502. Zammad not working. Solved. Documenting here

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#1

Running Zammad (3.4 and earlier) on Ubuntu 18.04.
For some at first inexplicable reason Nginx only said 502 gateway error.

systemctl status zammad
systemctl status zammad-web
systemctl status zammad-worker
systemctl status zammad-websocket

These were all green.

Restarting zammad gave indication everything was allright.
No mention of something special in the /opt/zammad/log/production.log
Checking netstat -tanp | grep 3000 revealed nothing listening on port 3000.
Checking syslog said:
Aug 3 09:42:24 zammad zammad-web-1.service[9809]: A server is already running. Check /opt/zammad/tmp/pids/server.pid.

Turns out there was a stale PID file /opt/zammad/tmp/pids/server.pid.
(This happened because for the first time in 20 years I actually had kernel panics with kernel 5.4.0-40 on Ubuntu 20.04)
After removing the file and a restart everything was nicely working again.

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#2

I have the same problem but it’s repeated every day (I suppose there is a daily scheduled restart of zammad-web-1.service). File and tmp folder permissions are ok. Where to find the problem? When I delete PID file and restart the service everything works fine.

[update]
I run ‘watch’ on the folder with PID and restarted the service manually and … zammad has deleted the PID file. The problem exists only on scheduled restarts.

I’m out of ideas ¯\_(ツ)_/¯