You are not affected by a stuck certificate.
Investigate whether your traffic is going via a proxy/corporate firewall which is performing deep packet inspection. These things do decrypt the traffic, then analyze it, and then again encrypt the packets using their own certificates. Plex clients won’t accept these foreign certs.
ISPs sometimes implement these as “enhanced internet security” and under similar names. You may want to disable them (if you are able to).
See also Important note about Chrome and Firefox Local Network Access
