Did you go to settings services and try logging back into Tidal?
If so, did the tidal login take you to a webpage to activate the link to Roon?
It “should” open up a website and ask you to login to tidal then give you this confirmation page.
Yes, I went to settings services and tried logging back. Symptom: After successful login on the webpage and activated link to Roon: no reaction. In Roon I’m still not logged in.
Later I stopped the Roon core app on the server and restarted it.
The Tidal functionality came back on the client after a while. Now it seems to work.