And adding to @ChuckPa here 
If you are a regular NAS user, and not into Linux, then try the following:
Install this: Create a share for Plex Library Directory
Then disable PMS from the QNAP AppCenter, and connect to the share named Plex
In there, you’ll find a directory named Plex Media Server and in that a file named Preferences.xml
Make a backup of that file, and then follow: Why am I locked out of Server Settings and how do I get in? | Plex Support to see what should be deleted from the file, IF the email address in that is not yours!
If it is yours, then check that your browser isn’t authenticated as another accound towards plex.tv
And remember after all this, to enable PMS again from the AppCenter
