My guess is that you accidentally signed your PMS into another Plex account.
There are Plex setting that don’t get removed by just installing PMS. You can try a full uninstall by following this https://support.plex.tv/articles/201941078-uninstall-plex-media-server/.
Or do a strategic strike and just remove the account information, https://support.plex.tv/articles/204281528-why-am-i-locked-out-of-server-settings-and-how-do-i-get-in/.
