Disconnecting the hdmi-cable for a few seconds made it boot.

This is a know issue I already had read about during initial setup (and didn't remember it). When I changed the connected client from a #cubox to my #Librem5 it hit me.

Thanks for the quick reminder and help and the interesting information about the development process of the #PiKVM!

It turned out that having the mentioned jumper No14 closed doesn't make much sense, because there seems to be no negative effect known and opening the jumper does rule out the hdmi issue I ran into.

BTW: Looking forward to Pi-Cast!