Actually, I was able to solve that issue. In my Windows 10 installation, I had set the "Region" setting set to "USA" (because I had made some tests with Cortana a while ago and forgot about it - never was an issue anywhere, yet, including the shop). Set it to German or wherever you're living and then the shop will work properly:
I want to upgrade to the Premium version, since the Microsoft Hello authentification is extremely welcome. However, the in app purchase doesn't work. It insists on entering an US address: My active windows account is properly set to my german home location, though, including the payment options. I am logged in with that account in Windows 10 as well: That keeps me from getting the premium version and is quite frustrating. I hope this can be fixed somehow. Thanks.