I did use a common Ground and I couldnt find any burned parts. It still runs its sketch when I power it via an external power supply on the +5V pin. But powering it via only the mini USB seems to fail. If I just connect the Nano to my PC via USB my PC does not recognize it. But if I plug in the usb AND a power supply at the same time, my pc recoginzes the board. So there has to be something wrong with the usb. Thats why i posted the Voltage i measured on the usb-diode on the backside of the board
regards