I unplugged the board from the power source (USB only).
On the unplugged board i pressed the reset-button and kept pressing it.
I compiled the sketch and hit CTRL-U, while still holding down the reset-button.
Once the RX-led blinks once (very fast) i let go of the reset-button, resulting in a completed and succesful upload. The sketch is performed without problems...
How does an LED flash on an unpowered board?