Bitteschön! (You are welcome)
So here is what I found out:
After opening the case I had a look at the mainboard.
There is just one fan header (regular 3 pin). It is located on the rightern side.
On the left side (where there is no fan header) there is no real possibility to mount a second fan.
close up:
So I removed the stock fan and and built in the NOCTUA NF-A4x10 FLX I not using the OmniJoin Adaptor Set.
Put all screws back together and powered the device.
All lamps yellow.... fan not spinning....
After taking all the screws out again: I noticed that the stock fan had the cables alligend from left to right: yellow, black, red and
not the standard yellow, red, black.
So without using the OmniJoin Adaptor Set (because I forgot about that) I took a needle and softly pressed against the pins of the noctuas fan connector and switched the red and black...
Fan was spinning.
Switch is operating normal.
Temps went up 2 degrees to 27C
The switch is really quite now.
Should be way more silent than 33db.