A hard rest does not work, nor does a 30-30-30 reset. The router is reachable and show the Dlink recovery page. Unfortunately, uploading no matter what firmware (original or dd-wrt) does not work. After a short moment of disconnetion, the router is up again, showing the recovery page!
What works is a serial reset as described in one of the posts.
It seems as there are leftovers in the nvram which cause problems...