This appears to be an incompatibility issue with older MariaDB version (5.x).
This is how you can fix this issue:

1. On your QNAP, in App Center, make sure to install MariaDB 10 (or newer).
2. Once installed, open it and follow the migration steps. Mine gave an error, but after re-opening the new version, it worked.
3. Next: Open the old MariaDB (5.x) and disable it:

    

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4. Make sure to take note of the port number as it has been used with MariaDB 5.x, which by default is 3306 - KODI will be relying on this number!
5. Go to MariaDB 10.x, make sure it's enable it, and set the port number to the one use in the old MariaDB.
   Also take note of the "domain socket" - we will need this to configure phpMyAdmin.
   Click "Apply" which will restart the MariaDB service.
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6. Next go to the Web share and look for the config.inc.php file for phpMyAdmin (/Web/phpMyAdmin/config.inc.php).
   You can either go their with your regular file browser, or QNAP's FileStation.
   Edit the config.inc.php file (when using QNAP FileStation: make sure Text Editor is installed and active on your QNAP - see QNAP App Center!) and add these lines:
   (make sure port number and socket filename match your configuration !)
   
   

   port: $cfg = '3306';
   socket: $cfg = '/var/run/mariadb10.sock';

7. Now stop phpMyAdmin and restart it in App Center. You should now be able to access your databases that have been migrated to MariaDB 10.x.
   Also: KODI 19.x now works properly again as well.
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