Ok, no panic. I don't know how and why, but finally it the router was recognized and the upload began. The procedure was like this:
1. Click around cluelessly in those ugly network configuration menus of MS Windows.
2. Wait until the network setting application crashes and you get an error window (not responding).
3. Right after the crash, the restoration tool will start the upload automatically.
Maybe it's to be recommended to go back to Linux and use tftp... seems to be easier. All in all I guess I failed in giving Windows a static IP at the right point in time.