Very high humidity in all rooms

Then all houses from the 60s would have to be molding away. Since this is not the case, there must (still) be other problems here.

Many do indeed. With the old windows, this isn’t yet a big problem, they provide continuous ventilation. In old buildings you quickly get cold feet, 20 degrees doesn’t feel like 20 degrees in a new building. So heating is usually turned up significantly higher. And then you slowly get to the point where you only have mold behind the cabinet and don’t see it. Here the OP replaced the windows and would now need to manually achieve x air changes per day. But he can’t, because almost no one can. With little wind, you’d have to keep the windows open far too long, in weather like today you don’t create any draft at all. The cold air from outside is unpleasant anyway. Ventilating after showering is nice and all, but actually you’d have to ventilate again an hour later to get rid of the residual moisture from towels and the shower. But most people are in the office then, and even if they work from home, hardly anyone does it. The walls from the late 60s and early 70s are also quite a disaster, energy was cheap and so was construction. So I’m not surprised that the previous owner already messed around with internal insulation there. Tearing it out was of course absolutely counterproductive and could only be topped by putting it back on oneself. Poorly done internal insulation molds very nicely between the insulation and the wall. The big question now is: should the cause be treated or just the symptom?

With the current temperature differences from outside to inside, especially in the mornings, you don’t need a "draft" to achieve good air exchange. On the contrary, drafts are often counterproductive because they spread the particularly humid air from the bedroom and bathroom throughout the entire house... I once heard about people who created an ice cave in their cold attic because the hatch was constantly open and located right next to the bathroom, where of course the door was left open after showering. Humid air, freezing cold roof... ice cave...

Whether the interior insulation installed by the previous owner was really done professionally, we’ll leave that open. We don’t know.

In the short term, he can only alleviate the symptoms anyway. Before you get a new kidney, you usually "run" to dialysis for years. Very few lie on the operating table for 2 weeks because they know someone who can donate a kidney.
The same goes here. Craftsmen who would install a decentralized ventilation system for him or insulate the wall, whether inside or outside, he won’t find anymore this winter.
So fight the symptoms: heat, ventilate, heat, ventilate, and so on.

And when you read "oil heating," it should be clear anyway that an energy renovation will be inevitable in the medium term. Unless "back to the 70s" Sahra allies with "back to the 30s" Tino after the 2025 election and pull a brand-new 6-strand oil and gas pipeline through the Baltic Sea out of the hat, which is about to go into operation.
I do like my gas heating, but I am realistic enough to know it will probably be the last of its kind in my life.

An open hatch to the attic is a shot in the foot anyway. We don't need to argue at that level, and that's not what this is about. This is about the fact that the OP's wall is moldy because, in his house, it is impossible to keep the air dry enough at normal indoor temperatures.

The topic "relative vs. absolute humidity" has already been mentioned, as well as the continuous generation of humidity. Therefore, I am once again bringing up the difference humidity-controlled fan in the bathroom. It then ventilates when it makes sense. I would place it a) in the bathroom and b) everywhere there is an individual moisture problem.

Of course, this does not solve the problem of the equally frequent and nonsensical wall construction, which has been taken to the extreme here by the removal of insulation during the insulation of the windows. This must be resolved as quickly as possible.

Thank you very much for the numerous feedbacks and tips. I have planned the following todos for myself.

[*]In the bedroom, test insulating the north wall from the inside with climate boards. [*]If that is not enough, install blown-in insulation and a fan. [*]Does anyone need a kidney? Want to insulate the house?

Regards, Andre

3. Does anyone need a kidney? Want to insulate the house?

Do you trust yourself to insulate the house from the outside (with friends or possibly classifieds)? Then you can do it for a "pittance." The material for a KfW55 envelope will be well under €10,000. Insulation, adhesive, dowels, mineral plaster, mineral paint, reveals, aluminum window sills.