I’ve already tried a few things, flashed the firmware with an earlier version, downloaded EZVIZ Studio and made sure RTSP was enabled. The standard EZVIZ app finds the camera, but will not connect to RTSP stream, it just times out.
Edit: I have figured out that the DB1C does indeed put out RTSP streams, as I can view them in VLC when I log on to the machine with the same subnet as the camera. The Camera is on a guest network, on a different router than the proxomox machine and associated home assistant VM (I decided to make it really hard on myself…). I tried to forward the ports (554, in and out, TCP and UDP) for the camera IP on the camera router, but so far it does not help. Still getting “In valid Authentification” when I try to connect the RTSP stream in HA.