@flinke-vent , is this an original Volla Phone or a Gigaset Device, cause its not allowed to flash non original Volla Devices with VollaOS (EULA) ?

@Pingu ....it should be no problem, to go back to VOS11 via SP Flashtool and after via Ubports installer to UT 20.04 Focal (the follow the screen)

Do you use UT via VollaBootManager from SD Card ? I had no problems with installing Waydroid via Waydroid Helper App a few days ago (Focal 20.04 Stable , Waydroid LineageOS Androoid 11)....You could install Waydroid also via Terminal