Help, I can't get my Maix adapter to work

Hello Maik, and hello to all the other experts,
I received and connected Maik's adapter, but it's not working. The car is a G4 TDI 90PS with AGR. With the laptop (VAGCom 303 demo) and car connected, the green and red LEDs are lit. During the LED test, the yellow and the other red LED blink, and the message 'Com3 LED Test OK' appears. However, during the test, an error message 'Error 5: unknown device' is displayed.
What am I doing wrong?
There are two potentiometers that you can apparently turn, but it doesn't seem to do anything (is only one rotation relevant?, or can you turn them endlessly?). I'm not an electronics expert . Or is there a specific procedure ? By the way, while adjusting, the red and yellow LEDs flash briefly.
I've also followed the tip of testing without the car connected, and the result is the same (even though no LEDs are lit, the LED test is OK), but the 'unknown device' message persists.
P.S. As Maik described, I connected the ground wire to pin 5 (signal ground) of the connector. However, Rainer's post about the adapter refers to pin 4 (car ground).
Could it be because of that?

Hi, please send me your private email address and phone number. Then I'll give you a call.

Regards,
Maik.