There comes a short late night message.
Facebook itself doesn't allow for logging into their system from an application itself. Instead every user is required to take the detour to a facebook login page and log in there.
As this procedure is not convenient at all in our mobile application, we decided to bypass the Facebook login procedure and emulate a browsers behavior inside our application.
After hours of investigating the HTTP communication between browser and Facebook server, we finally managed to obtain our first session key late the first day!
With this session key we are know able to communicate with the Facebook API as if a user would have logged in at their web page.
A big big hurray to the techy team! :-)