Hello everybody,
I connected a 12V battery to the Vin and it worked without any problems. I did not measured the voltage of the battery around that time. Later, I charged the battery using a battery charger (same I use for my motorcycle). After that I connected the battery again (and for a short moment shorted the plus/minus on my connected breadboard .. 
..). After this whole fiasco I measured the battery and it was 13Vdc.
Now the arduino is still running fine using the usb connection. But when I connect the battery it fails. The I2C display still lights up but it only lights up completely. Not displaying characters. If I connect a 9 volt power supply to Vin everything works fine also.
Also at one point I connected the battery while the arduino board was still connected to my laptop. After that immediately my laptop shut down (insert sweaty palms here). Luckily laptop still works 
Won't do that again..
Anyway my limited experience with electronics says to me that the voltage regulator is busted. But (correct me if I am wrong) usually this stuff either works or it does not at all.
Anybody got some tips/hints or experience with this?
greetz Piet