Hi, just the rest of my story:
I opend the Phone and realised after some time that i cant solder the Button on my own. Its way too tiny. So i let a fried of mine who solders micro-electronics as part of his parttimejob. And looking at it, he done a good Job. But the transport or me did break some other part of the phone since it didnt start after i reconstructed it.
At that time the fairphone support, which i contacted after creating this Thread finally replied (they hat much to do and not enough time for everyone because of the FP2). So i told them the story above aswell. So they asked me if i want to participate at their fairphone buy-back program, which i then did.
So i send my phone to Amsterdam and after 2 weeks or so i got some cash for it on my bank account.
P.S. after reading and watching some videos about the replacement of the button i can say with a high probability that the button was the problem since other phones with the same button have the same never ending rebooting problem, which can be solved by replacing the button.