Have a little box to play around. That box was still on OMV2.
As I wanted to test OMV4 I decided to Upgrade it from 2 to 3 to 4.
Went pretty good - just the last upgrade step - from 3 to 4 had a little problem.
After a reboot I got the "502 Bad Gateway" error.
After a quick look in the logs I discovered that nginx is trying to access php-fpm on a wrong socket.
The file /etc/nginx/sites-available/openmediavault-webgui references to
after the upgrade. But it should reference to
Don't know where this happened on that upgrade path - but as OMV3 was accessible pretty normal I think it happened on the update from 3 to 4.