Hi @Markus_Menasci,
First, check that your account is still active, as suggested here . In the past we have seen that you may be able to login to the TIDAL account in the TIDAL app, but the subscription itself may not be active therefore cannot connect to Roon.
Are you able to play TIDAL content outside of Roon from this Core?
If you’ve confirmed that your account is active, stop RoonServer on your Core, find your Roon database and delete the tidal_account file (Database > Registry > Core > tidal_account). Once you do this try to login again.