Thx,
I power it from USB only, so luckily no overvoltage to the nano or chip.
I have disconnected the Nano from the board and connected it via USB to pc. The pc does not see it and after a few flashes of the Nano pwr led the led stays off.
I did notice something else: If I power it from a separate 5V regulated power supply, the pc sees it over USB and I can flash with the blink sketch, which works fine.
However the main question remains: Did my circuit design cause this?
Any ideas?