try sign out and sign in again - on the server settings general page
Sign Out and then reclaim
if an error is returned, get the logs and upload the zip here (assuming debug logging enabled)
See
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
https://support.plex.tv/articles/200250417-plex-media-server-log-files/
