Plex support articles do not help

Plex Media Server NAS & Devices
3

No idea how it happened but what you show here can happen when PMS doesn’t shutdown cleanly. (DSM upgrade or otherwise)

Did you also update DSM to 7.2.2 ?

Fixing it is trivial.

  1. Uninstall the package with the “KEEP” option
  2. Reinstall the same SPK again
  3. HOWEVER – Use the Plex Claim Token option
  4. Click 'Get Plex Claim token"
  5. In the tab which opens , COPY the token
  6. Return to the installer page and PASTE the token into the box.
  7. Click NEXT - NEXT - DONE

PMS will reinstall AND reclaim it for you again.