Hi everbody,
i solved the issue by myself (by chance)!!
I didn't have to return the nissin because it was just a matter of one long press on the LED next to the ON/OFF Switch.
I haven't found an explaination of this behaviour in the manual but if you press the LED for approx. 2sec. the LED next to the Mode Dial starts blinking.
After that the Nissin fires only trigger flashes in M Mode.
After i hit the button for 2sec. again the Mode Dial LED was shining permanently - and the Flash works as expected in M Mode!
Thanks Adam for you support!!
regards
Thomas