I can make it a little more strange
I removed the SD-card many times to reinstall the system using Linux, because it wouldn't boot. So I became angry and removed the pcb to check if there maybe was a flaw in the hardware. Maybe a soldering was bad, but I found noting
. So I connected the power and pressed the powerbutton. And believe it or not ... the system started!
So what caused it to boot? I held the board in my hand and my thumb was pressing the SD-card a little. Because of the little pressure the SD-card makes better contact with the board.
I have now folded a piece of paper, which is laying between the top-side of the SD-card and the top face of the holder, thick enough to remove the SD-card easily, but it creates enough pressure that I can boot the system without any problems! I have booted now 4 times into the os-selection to boot AI-os, Android or Ubuntu, and it works!