This really is one nice looking platform! Thanks for the link.
I have come to the conclusion that I am beyond help - guess its my own fault for buying them without prior testing. However I needed them fast and they where the only ones my supplier had 25 pieces available at that moment :-/
Reading that someone agrees with me already makes me feel a bit better though 
I have this vague Idea of asking the Arduino Team for a refund or an exchange, but I am not sure how to go about it though. I mean... it is almost 500€ worth of material which is basically not performing how I expected it to. I cant really claim it was "false advertising" though as they do document at least most of its little quirks. Still I somehow feel cheated out of my money. Eventually I will be buying new boards and these will be left to collect dust or maybe I can sell them over e-bay at a loss and with a bad conscience.