Step 1 might be updating your server.
Based on the version of Plex Web you’re using on this server, it’s on version 1.22.x which had been released back in April 2021.
The latest public release is version 1.24.2 from approx. 2 weeks ago.
If that doesn’t solve your issue it might help to take a look at your server logs – while the server is running you can access those through the web app, otherwise you’ll need to manually grab them from the QNAP’s file system.
Edit:
It might also be worth to update your player app. Whichever app that is… it appears to be outdated.
- the latest release of the regular desktop app (Plex for Mac / Windows) is version 1.34.1
- the latest release of the old HTPC app (Plex Media Player) is version 2.58
- the latest release of the new / future HTPC app (Plex HTPC) is version 1.5.1
