Okay, this is/was the problem:
the cloning DOES start, but it is really slow.
There is no image file visible in /images/dev for a really long time (20-25 minutes).
Thus, I can live with the black console 🙂
The console wont show anything, even with serial-stuff enabled in the pxe-config file.
i dont care any more, i am happy that this thing (an embedded system without vga-card) can be cloned :)))