Even though this is absolutely without meaning - it works now. I checked and changed and rebooted my router many times during this process. But this morning I looked for a new firmware and upgraded the router. Unfortunately this ment that all settings were lost.
I then reconfigured the router as before - and bing - there it was
This once again shows that working with IT is a combination of science and woodoo. Falko - many thanks for your effort. And for other readers I hope that you might find some usefull knowledge in my problems.