I had a similar issue of no gui after upgrading to 1.12. Shares and ssh worked, it looked like just the webgui wasn't working.
I tried all the steps above and from other threads where people had a similar issue but no cigar.
apache was running so I did the above to remove apache but still no gui, but not only no gui no website at all, connection refused.
nginx was running but the above netstat command didn't have any output.
omv-firstaid, reinstalling omv and nginx didn't help.
I did some googling on nginx and found that the dir /etc/nginx/sites-enabled was empty. I linked /etc/nginx/sites-available/openmediavault-webgui to /etc/nginx/sites-enabled/ and the gui worked.
hopefully this helps someone with a similar issue.