Humidity permanently too high, mold formation, cause?

Aside from that, are the wall values okay? So no indications that moisture is penetrating?

Is that normal for the shower and the floor tiles?

You are going in circles and looking for some moisture damage because you do not want to accept what the people here have advised you several times. You are supposed to heat and ventilate everywhere. Yes, also heat the bedroom.

I am doing that too. However, I was also advised to check if water is seeping in anywhere.

I am doing that too. But I was also advised to check if water is seeping in anywhere.

You would notice something like that immediately. From your posts, one can directly read an incorrect heating and ventilation behavior. How long has this incorrect ventilation and heating been going on?

Actually, we have always heated and ventilated like this since we have been living there. So about 10 years. However, it is quite possible that there were no problems in the first few years because we were out of the house during the day, so no moisture was produced.

Well, of course that does not apply to the bedroom, it has always been used like it is now.
Now we often work from home, which means there are two people present plus a dog, and more cooking is done than before.
I have been heating and ventilating a lot over the past few days and am trying to solve the problem.

However, it is currently foggy and dull again at 8 degrees during the day, and the humidity only drops to 50% even after ventilating. It was already better on the weekend with sunny and cold weather.

So, I have now pulled the bed away from the wall and there was no mold behind it. That's already a good sign.

The exterior wall in the bedroom shows a humidity level of around 55-60% on the moisture meter. Not great, but not dramatic either, I am currently drying it with heat.

However: Behind my nightstand (which is placed exactly in the corner, two exterior walls at a 90-degree angle) there was some mold again. The area is not dramatic, I will remove it. But there I measure around 80% on the wall and that is too much. When I go right into the corner, at the bottom, I measure values over 90%.

Now my question: How do I find out if moisture is penetrating from outside? Or does it simply accumulate there because it is in the corner of two exterior walls and at the very bottom, thus probably the lowest point and the coldest, and without air circulation due to the nightstand?

My idea now was to move the nightstand 10 cm away from the wall on each side so air can circulate. Remove the mold. Then dry the wall (possibly using a fan heater) and then wait a bit. If it goes back up to similar values, it will be coming from outside. If it doesn’t rise again, it is indoor moisture.