Stop plex, go to the folder in the path below and remove the certificate from the folder, *.p12 (Your cert may not be named cert-v2.p12, it may just be certificate.p12), and restart plex so that it pulls down a new certificate. If that doesn’t fix it, may need someone from plex to refresh it on their end.
/Users/<username>/Library/Caches/PlexMediaServer
If you don’t see that Library folder in your user directory, show hidden files/folders by pressing Command + Shift + “.” .
