As titles says I had to reinstall and restore DB.
All clients (everyone without exception uses Gajim 0.16.5) can log in to their accounts after accepting the new SSL cert.
I can see them active in the server roster.
But all contacts in their client rosters now have an error "remote-server-not-found".
I tried installing Psi and it has the exact same issue, so this is not a Gajim thing.
The accounts are registered using LAN IP of the server to connect. Tried the FQDN host.domain.com as well and that did not work either.
How can I clear these errors and let the users communicate again?
I fixed that by changing the Server Name in the Server Information/Edit Properties from the host name of the server machine to the domain name of the of the LAN.