status=“Plex Media Server is currently running startup maintenance tasks.”
It is expected when updating to a release with many database changes.
EDIT: Also seeing Error binding acceptor: Address in use in the log files.
Something is holding the Plex port open.
Suggestion:
Stop Plex Media Server.
Reboot the NAS.
Start Plex Media Server.
If you still see the 503 maintenance message, wait. Check every 30 - 60 minutes (refresh the browser window or close/open a new window). Depending on the size of the db and the speed of the CPU, it may take minutes or hours to complete.
I know in the linked threads I wrote to not stop/restart the process. However, you already restarted PMS 4 times in 13 minutes, so one more won’t hurt.