Can't log into App Store

Hiho,

I just got my new MayBook Pro and I was transferring my data from my old one to the new one. Also I was trying to load some Apps and updates via the App Store. So far so good, I click on "Update" on iPhoto and the prompt comes up telling me to enter my Apple ID. So I enter it, hit the "accept" button... and.. the loading animation on the left comes up.. and never goes away. I tried waiting for over 5 minutes in this state and nothing happened. Funnily enough I can sign in everywhere else this this ID. Here in the boards, in the iTunes store, wherever I want. But the App Store hangs. It also does not matter how I get the login prompt, if I try to set the account via the Store-section in the menu bar the same thing happens.

Any clue what could be wrong?

Just to be clear: I did it often enough to be sure I entered the right ID and password, I do have internet connection AND when I enter the password wrongly I immediately get a notification that the username or password is bad.