To add to @dbirch’s comments, there is no need to edit preferences.xml to unclaim/re-claim.
Instead, uninstall/re-install Plex Media Server using Synology Package Center. See FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out).
If the “ghost server” is still hanging around, remove it from your list of Authorized Devices. You may also need to unpin associated libraries in your clients.
