Problem solved - no clue what caused the issues, but here’s what we did to get it up and running again.

• VSphere Restore to backup from Thursday, last week (when GUI was still online)

• ipv6 still disabled for the system

• edited zammad.conf and commented out the following lines:

listen [::]:80

listen [::]:443 ssl http2;

Restarting nginx.service - succesful

Zammad GUI still says bad gateway, website not reachable

Checked running services and noticed that zammad-web-1.service failed to start.
Restarting the service also failed.

Checking the status of zammad-web-1.service:
Error pointing to service.pid file

Deleted that file and restarted the service - successful
Restarting nginx.service - successful

Website reachable

We then restarted the whole system and everything seems working again.

As the last server update was done quite a while back we have no clue what caused that issue. System hasn’t been rebooted since.