Oh, short notice:
The German Telekom seems to have hired the same "web designer" for the administration interface of their WiFi routers, too. I just tried to change basic settings like IP address and WPA password, which turned out to be a quite complicated task as the admin interface was overly filled with useless JavaScript stuff as well as a couple of hard redirects to the IP of the router.
I imagine the "non-functional requirement" the code writer had to fulfill was something like: "Make remote administration of this device as complicated as possible". This is especially sweet if you want to access it from a distance of a few thousand miles...
Anyways, with a few tricks I managed it finally.
Thank you for wasting my time, German Telekom...!