can't upload any sketch to Sparkfun Pro Micro 3,3V

Hi,
this is my first attempt to use a Sparkfun Pro Micro 3,3V.
I have the current Arduino IDE 1.8.3, have installed the Board Library from Sparkfun and installed the USB drivers on Win10.
The IDE has selected Sparkfun Pro Micro as Board and At Mega 32U4 at 8Mhz (3,3V) as processor
The Pro Micro 3,3V ist connected over COM13 (disappears if I remove the board from USB)

So everything should be fine, should it?

When I upload a sketch, it compiles and uploads (TX and RX from the board are blinking alternatively)
But it gets stuck at that stage and TX and RX blink forever.
Even when I reboot the host, it keeps blinking as long as USB provides power.
If I short the reset Pin the blinking stops, but the sketch is not loaded.

Has anyone got a clue how to get the sketch uploaded?
Thank you

You can get more information on what's happening during the upload by turning on verbose output (File > Preferences > Show verbose output during: > compilation (uncheck) > upload (check) > OK).