I had my car serviced at a workshop in Coburg. I actually wanted to have the air conditioning system disinfected, but the workshop owner advised me that on my next trip, I should turn the fan to setting 4 and turn on the air conditioning and recirculation mode. This will allow the heat from the engine coolant heat exchanger to dry out the air conditioning heat exchanger.
It really made a difference for me, although it seems my interior filter definitely needs to be replaced, because it smells even without the air conditioning. It's supposed to be changed every 30,000 km, but maybe 15,000 km is better.