Thanks a lot! This works now!
I connected the FTDI cable this way (more or less same as above :-) GBoard FTDI
NC (not connected)
CTS# (not connected)
GND (not connected, one pin for GND is sufficient)
It seems to be important to set the UART via jumper correct: http://arduino.cc/forum/index.php/topic,146708.0.html
The only problem I see atm is: When I plug in the USB cable, then click upload, press reset on the board all works fine and the sketch is uploaded. But after a second trial uploading an other or modified sketch the IDE reports "Serial COM already in use". After unplugging the USB--waiting a moment--insert the plug again--it works fine. But I have to do unplugging. Any idea how to avoid this?