You should be able to go into server settings and claim the server.
If that does not work, see FAQ 15: Claiming & Connecting to a troublesome Synology server.
Preferences.xml will be in the /volume1/Plex/Library/Application Support/Plex Media Server/
.
Method: Last ditch effort
- Stop Plex
- File Station
- Navigate Library / Application Support / Plex Media Server
- Rename Preferences.xml → Preferences.xml.old
- Start PMS
- Open http://ip.addr.of.syno:32400/web?secure=0
- Begin normal setup EXCEPT do not create new library sections. SKIP this.
- Proceed normally through to the dashboard.