This was for the server itself requiring re-authentication with Plex for the server claim token, rather than client auth. Some situation arose where the claim token was no longer valid, expired, unsure, and the server was locked out and local media inaccessible until ISP outage resolved and could login with Plex account (2 weeks due to fallen tree). Not ruling out a config issue. Was a couple of years ago now, so bit fuzzy on the details.

This. I’ve had a couple of situations where we had an ISP outage and for whatever reason Plex Auth had expired and needed to connect to their servers to regain access to local media. The first time it happened I was pissed off. The second time it happened I installed Jellyfin and never looked back.