Ok so for the very time now I lost all coins due to some weird error: the wallet didn't synch anymore and I deleted everything except wallet.dat. I also made a copy of that wallet.dat file. When I launched the client again, it didn't start and it told me that the wallet.dat file is damaged and turned the file into a "number".bak file.
I tried to install everything again and put the wallet.dat into that folder. It is now a zero balance. Interesting... I did it several times now to check whether I am doing something wrong, but as I said, never ever had that problem before.