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: 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.