Stop and restart the server.
Use the hosted web app in a regular web browser (and not in a locked-down browser like Brave etc.)
Reload the web app after restarting the server (F5).
Or inspect them yourself. Take a look at the Plex Media Server.log file and seek for messages about database corrupt or malformed.
If you find these, you may have to repair your database. https://support.plex.tv/articles/repair-a-corrupted-database/
Try disabling IPv6 in the settings of Plex server. (stop and restart the server afterwards)
Your log shows access by IPv6 during the failing configuration attempt.
Do also try to temporarily downgrade your server software.
You should be able to find older software versions in the /Updates subfolder of your Plex data folder.