No HDMI Video Output

I am trying to set up my Rock Pi 4 B wit either Debian or Recalbox. The Pi4 seems to boot up completely as I can enter the Pi via SSH/Putty and it seems to work. On Recalbox I can enter the Web Interface as well and it seems to work. But I do not get any HDMI output.
Does anyone have an Idea what this could be?