well this might be a bit out a stretch but is your data center on a static or dynamic ip.
are you accessing your plex Via app.plex.tv or localhost:32400
have you ruled out any double Nat is your server on a static IP. port forwarding is also needed in the router and all hops to your server from a publicly accessibly IP.
This is message i get if I access it via local hostname
This server could not prove that it is mediaserver ; its security certificate is from *.f1c2bb84a23340b8b08557b853404930.plex.direct . This may be caused by a misconfiguration or an attacker intercepting your connection
I am running my server on Ubuntu Server (CLI Only)
The certificate is pinned to the … plex.direct FQDN.
If you use a custom host name for your Plex server, the certificate is of course not valid for it. Use either the IP address or the long plex.direct FQDN.
Or even better, use app.plex.tv since it will always have the latest updates.