In the end I sent it back to ereader-store.de and they did the reset again with the 20120309-trick
. I did it myself multiple times with two different SD-cards but this wasn't enough. I won't try to update any more my ereader before my exams though.
Thanks to booxtor & co. for the support!
@johey: I'm happy for you that you managed to solve this by yourself.