You’ll probably the victim of Thecus not updating their firmware.
PMS 1.14.1 is quite an old version (dating back to late 2018). The release announcement for version 1.15.0 mentioned this dependency:
Plex Media Server 1.15.0.647 is now available to Plex Pass users in the Beta update channel
This version contains numerous changes and a lot of invisible behind the curtains stuff. There a few caveats and some missing builds at this point:
Notes:
- Thecus Builds for Thecus devices will not be available at this time. Thecus is working on a firmware update that allows the new PMS to run on their devices.
From all I could find, Thecus is still due to deliver this update.
There’s a number of users who apparently raised tickets with them but so far to no success. Some of that is documented in another thread… read is quite depressing 
At this point your best shot might be to keep the NAS for storage but move the server itself to a different machine (e.g. a mini PC like Intel’s NUC or the Nvidia Shield).
