Sorry I have moved this post to the correct area of the forums: https://forums.plex.tv/discussion/223972/404-not-found-when-upgraded-to-latest-version-of-plex-media-server-on-qnap#latest
The only thing I can think of is how you may be trying to connect.
Are you using the start menu or task bar? I had similar issues by trying to bring up the PMS by the start menu when I should have been using the task bar. Try connecting with the task bar icon by right clicking the Plex Icon and then clicking “open plex” It won’t connect you with a left click to this icon on the (probably hidden Plex icon) task bar.
