Lost access to my Plex media server, it may have been deleted

Plex Media Server NAS & Devices
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If you’re using DSM 7, the SPK installer allows you to reclaim the server even if you’re on a different subnet / otherwise locked out.

If I may suggest –

  1. Don’t use app.plex.tv to open your server for now. Use the Syno’s LAN IP address http://lan.ip.of.syno:32400/web style.

  2. To reset the ownership credentials for your server, on DSM 7, refer here.