Can you please confirm that you are able to login to TIDAL using the web player (https://listen.tidal.com)? I would also try rebooting your Core and networking gear to see if that helps, please let me know.
Thanks for your quick reaction and I reinstalled the Roon App on my QNAP NAS and Tidal is working again.
So that did the trick. thanks ! (BTW I was able to connect to Tidal directly through the desktop app on Windows 10 and the web player. Only logging in to Tidal through Roon was not working.)