Stop and restart Plex server.
Ensure that it has unfettered access to the internet. Check any firewalls or filters which could potentially interfere with this.
If this doesn’t change anything, try the next step:
stop Plex server
locate the Codecs folder inside the plex data folder. Remove all subfolders it contains, but leave the file .device-id in place.
start Plex server and wait about 5 minutes
then try again
