Ventilation in summer without controlled residential ventilation is problematic

I haven't managed for weeks to raise the humidity in our apartment even close to 40%, no matter when I ventilate. I explained it to myself that the air is generally dry due to too little rain.

no, in summer it is more humid, in winter dry. Your feeling is of course not wrong. It actually feels the other way around because of the temperature.

I thought that you are not allowed/should not turn off a controlled residential ventilation system because then the pipes would become contaminated? Our last controlled residential ventilation system didn't even have an off switch.

also wrong. Of course you are allowed to turn it off. However, it is designed for continuous operation and that is why there is no clear On/Off button

When it's cooler outside than inside, ventilate, it's that simple. PS the controlled residential ventilation users turn off the ventilation during the day.

Even if the air outside is more humid?

Even if the air outside is more humid?

Yes...
Download the app "besser lüften," it is self-explanatory.

Yes...
Just download the app "besser lüften," it is self-explanatory.

Unfortunately, I have iOS and it's not available there. I downloaded a similar app. That clearly shows my problem again. Currently, it doesn't get cold outside, not in the evening, morning, or night. On top of that, there is high humidity outside, so what to do?

Leave the windows closed during the day. Still air out briefly in the morning and evening; 14 degrees and 80 percent humidity outside are still drier than 22 degrees with 58 percent inside. Ventilating makes your indoor air drier. Even if that is hard to believe....

Our house is quite simple in that regard: when we are at home, the windows are open as long as the outside temperatures are not too cool. We simply love the outdoor air that smells of forest and plants. The location and architecture make it possible without any appliances that despite a high insulation standard, we do not fall into the trap of trapped heat.