I think I found the solution on reddit and also the B&O hotline confirmed the issue.
If you factory-reset the BV Horizon, you basically brick it. Google stopped supporting the BZ Horizon, so you can no longer log into Google and also not use any smart device features.
B&O recommends to connect a Apple TV instead. This does not solve though that Spotify-Connect and native Chromecast no longer works.
Seems I now have two devices (Horizon and Moment) that are bricked due to B&O no longer updating the software or not renewing licenses. In case of the Horizon, this is quite an early decision as it was still sold 3 (?) years ago...
Lesson learnt!