I’m a bit confused… do you run 2 Plex Media Servers (1 on the Syno, 1 on your Windows machine) or are you using the Syno only as storage for your Windows Plex server?
If it’s the latter, you can keep your Syno running – just make sure the Plex Media Server app on Windows is actually stopped. If you keep getting that error after stopping it from the task tray you could double-check/verify if Plex is still running from within the Windows Task Manager.
Edit: PMS 1.28.2.6106 is an awfully old version (this version was released in August last year)… you might want to consider upgrading your server.
