Tap water smells musty

...and your expectation is that we consult our crystal ball?

No, but maybe there are people here who know the problem

Is there only this one fitting in this bathroom? If not, it can neither be the system nor the supply line. Did this suddenly occur? Has this tapping point always been used? Please describe the situation more precisely.

Is there only this one faucet in this bathroom? If not, it can be neither the system nor the supply line. Did this suddenly occur? Has this tap always been used? Please describe the situation more precisely.

There is only this one faucet, Grünbeck has already converted the system because it is suspected that the water supplier chlorinates, which the system also does, and you can taste it. But neither in the kitchen nor from the fridge or shower does it smell like it does from the bathroom sink.

The water filter is also a Grünbeck with a cartridge inside (which I will soon replace again).

We have noticed this for a while now; sometimes it’s more, sometimes less. The laundry from the washing machine doesn’t smell like that either.

The hot water supply runs via a transfer station with district heating (water).

It's only this fitting, Grünbeck has already switched the system because it is suspected that the water supplier chlorinates, which the system also does, and you can taste that. But neither in the kitchen nor from the fridge or shower does it smell like it does from the bathroom sink.

You keep speaking in riddles.
And now it smells from the sink? Sorry, I'm out.

Doesn't the drain maybe just stink?