Homeassistant Installation impossible - blinks, nothing more

Dear community,

I have been trying this again and again step by step according to this guide:

Flashing the image works flawlessly.
I connect the pi to the router, connect power. Blinking in the beginning is a bit random.

After a while, it settles to seven times green, then pause. Red always on, lan lights always on.

The pi is not visible in the lan devices of the router, also not via homeassistant.local:8123 or the other links.

Attaching a screen gives no display output.

Why so? Any help is appreciated