Thanks to Jacques!
I got my Kobo running again with his virgin Image.
it worked like this, make a backup of your kobo sdcard in linux with dd.
dd if=/dev/sdX of=/home/youruser/backup.img
in the image you will find with a hex editor in offset 200 (before any partition starts)
the serialnumber of your kobo. copy this.
If you have the virgin image from jacques, insert your serialnumber in his image.
write the image to your sdcard
dd if=/home/youruser/new_with_serial.img of=/dev/sdX
voila all fine again.
if you use windows, you can use this software (
http://sourceforge.net/projects/win32diskimager/) to read and write the image.
Greetings
Grunge