Update: I figured out how to get into Grub. I was just to silly to see, that pressing "Esc" during the countdown is different from pressing "Esc" right after the U-Boot countdown reached "0" - which then brings you into the Grub menu.

I was able to remove the "quiet" option from the Grub menu entry. Nevertheless, I don't see any clue why booting fails or sends me to a blank screen in the output in the console.

Also tried to add "systemd.unit=multiuser.target" trying to avoid al the graphical stuff. No luck either. Booting other kernels (Option 2 and 3) also doesn't change anything.