Search for the cause of the mold

At 20 degrees and 60% humidity, the dew point is at 12 degrees and thus above the surface temperature of your walls in the kitchen and bedroom. This means: Water condenses on your walls, they become damp and form a habitat for everything that floats around in the air (e.g., mold spores).

: Google "calculate dew point". The first result will give you a dew point calculator. Then you can follow 's post.

In the bedroom, it always gets quite humid overnight. This cannot be avoided with two people sleeping. That way, you are practically inviting mold on cold walls.

Your apartment looks like an old building. What is the landlord supposed to do about mold here? Insulate the facade of your apartment as well? Apply a fungicidal coating to the walls?

As already mentioned: Heat more -> room temperature rises -> relative humidity decreases because warm air can hold more water.
If the humidity stays the same with you because you think "if we heat more, we can ventilate less," then you are likely making the problem worse because the dew point rises and thus the humidity condenses on the wall not at, for example, 12 degrees but already at 13 degrees.

Possible options:
- Breathe less
- Get rid of houseplants
- Ventilate more
- Heat more (and do not ventilate less at the same time).
- Get a dehumidifier with a hygrometer and run it continuously.
- Install (have installed) a ventilation system.
- Move out

Isn't there a general formula for that? So the thermometer shows 20 degrees inside at 60% humidity and 0 degrees outside temperature = little dew, no mold Or 18 degrees inside at 70% humidity and -3 degrees outside temperature = a lot of dew, risk of mold

Isn't there a general formula for that?
So the thermometer shows 20 degrees inside at 60% humidity and 0 degrees outside temperature = little dew, no mold
Or 18 degrees inside at 70% humidity and -3 degrees outside temperature = a lot of dew, risk of mold

See post 7 ==> you have to calculate that, because it depends on the temperature of the wall. Warm walls (insulated houses) are less susceptible than old buildings.

Read this through: I believe the dew point issue is likely the cause and solution to the problems. That the radiators are awkwardly placed, of course, adds to it.