Hi Alexander,
can you try it with an HDMI Display? One of the mentioned prerequisites is the connection with an HDMI display...
In principle, DSI should work also (as it is the case for RasPi-4b with the official 7" touch display), but we have not tried or tested a DSI display on that board. You can check the error codes from DRM to get more details about the issue.
Best regards,
Manfred.