I did contact the Dlink support and was successful. The following procedure did solve the trouble for me:
I had to download the 1.14 FW directly from the website and flash it via GUI. Then 7s press of the reset button on the back of the camera. After that, I was advised to used the setup program again. Still the tool can't find any wifi networks, but selecting the ethernet connection I could establish the link to mydlink again. After I finished the setup I had to manually change via GUI from LAN to WIFI. This worked.
Basically I did this whole procedure already before, but only with older firmware 1.12 and 1.11. Never with the 1.14. When I downloaded the 1.14, I recognized that the filename was 1.14.02. Maybe there was also a first shot of the 1.14 which caused this trouble. Reverting back should also have solved the issue, I guess. But didn't. Still I don't completely understand the cause. Anyway the setup tool must have an issue too, as even now it doesn't show any wifi networks.
Thanks to everybody who tried to help !!!