Succes....
After messing a whole lot with it again today, I finally got it working.
First I cut a controller cable (guess what my neighbour, which plays a lot on it, said....)
Tried soldering it like an extension cable
Same error

Tried soldering the usb "port" to the backside of the print in which the Xbox USB cables is mounted
Same error
Got mad.....

Cut the USB cable on the keyboard, and soldered it directly to the little white plastic thingy, which goes into the print in which the Xbox USB wires are connected.
IT WORKED...
So my guess is the cable was too long....
So finally - Mandrake is running on my Xbox...
COOL..
Thx for the help all of you (And hope this message will help others to get it working)
Regards
Smiley_dk (Which still needs to get his mouse to work)