Too high humidity in the apartment. 60-70% in winter

Which device would you recommend?

Regards

Just google it, there are companies and hardware stores that rent something like this. But there are also devices in the hardware store for 15€ that are not as accurate but are sufficient to detect moisture in the walls. It is possible that the dampness penetrates inside through hairline cracks in the exterior wall. However, the cause can also be inside, water pipes or heating pipes, you don't necessarily have to see anything.

Clarity is only achieved by measuring all surfaces in the apartment [Messpunkte im 50cm Abstand]

What kind of waterfall is supposed to be flowing inside the walls that this would be a cause for rapidly increasing humidity?

Well, for weeks now, the outside air has simply been very humid due to rainy weather.

Yes, relatively humid. However, since cold air can absorb significantly less water than warm air, the absolute humidity is quite low and is easily sufficient for ventilation.

So here in NRW we have 10 degrees with 90% rel. humidity forecasted for today. That would be over 50% rel. humidity at 19 degrees. That’s what I’m getting at. Depending on the place of residence, we don’t have typical winter 2-3 degrees with low rel. humidity, but rather wet autumn weather.