[lkchris]

It appears you state that only one headlight works on low beam and both work on high beam.

First suggestion is burned out bulb. The bulbs are unique between low and high, although same H7 spec, of which there are four.

Using hard to follow language, it's suggested moving a known good bulb to the connector now connected to the non working bulb. A good suggestion.

It's been the German standard for decades that the wire to the bulb for low beam is yellow and the wire to the bulb for high beam is white. Ground is always brown.