No hdmi video output when no ethernet is connected to a raspberry pi

I had a problem with my raspberry pi: when no ethernet cable was connected, there was no video on the hdmi connected monitor. With an ethernet cable plugged in, the monitor would show the boot messages.

It turns out, that the hdmi cable was too cheap. There are different ground wires in an hdmi cable. Some manufacturers united the ground pins all on one cable. That sees to be the cause.

With another (better) hdmi cable, the output is also there without an ethernet cable.