@pascal Hey Pascal.
I recommend you to open the bootloader manually. Works best on a Linux system. If you opened the bootloader, the installation of UT with the UT Installer should work.
fastboot flashing unlock
is the command you should use. Keep in mind that the phone must be set into the fastboot mode to execute this. If the VollaOS is installed you have to close the bootloader with
fastboot flashing lock
This had helped me to switch from UT to VollaOS