UPDATE!
While I did not managed to get my Kobo back, I put it away for a while...
But ... this afternoon I tried again. And it appears I was not flashing the good FW on my SD card!
I have a v1, not a v2 ^^
I did not see the correct serial number. It is located on the side of the Kobo reader.
Now it's working again
I just have a wrong touch screen behaviour on a certain screen area. The touch seems to have an offset: When I touch the screen, it selects 2cm on the left. I don't know why ...