Thank you Holger, for looking into my problem. Sorry, that I didn't mention using LVM with full disk encryption. I tried several times to boot and type the unlock pass phrase without seeing any output, but unfortunately, this didn't bear any fruit.
And I tried to install Debian with LVM but without encryption and ended up with the same behaviour again. So not displaying the LUKS prompt doesn't seem to be the problem. It sits deeper. I will try to not use LVM in my next attempt.
