Now I think the cardreader is the problem.
I turn on the machine.
place an SD card (there is no laptop connected).
select home and then print from SD card.
The machine switches off spontaneously and one second later ON again.
I remove the SD card.
in the menu "printing from SD Card"
Without SD card he reveals the SD card files (which are not there).
To make a long story short:
the card reader switches OFF spontaneously even without printing.
I would like to exchange the defective card reader for a new one.
What is the procedure?