[tomi1988]

Rovio just wrote back:

Rovio Support Team, Jun 30, 08:34:
Hello,

and thanks for contacting us!

We have received some feedback about this issue. It seems that you have not verified your Rovio account. We have now verified your Rovio Account manually. Could you please be so kind and let us know if you now can start Angry Birds Friends correctly?

Best regards,

Johan
Rovio Support Team

This solved my problem. So if anyone is having the same issue, verify your Rovio account!