Very high humidity in all rooms

Then only blown-in insulation remains and raising the room temperature to 22-23 degrees, and keeping furniture away from the exterior walls.

Exactly, keep the temperature in the affected rooms permanently high AND keep the doors closed, track humidity and, if necessary, open the windows briefly until the humidity is down. And no plants in the affected rooms.
It is also helpful to always know the outdoor temperature and humidity so that you can see whether airing out is actually effective. There are tables online that compare temperature and absolute and relative humidity.
The decisive factor is that the north wall becomes warmer so that nothing condenses there anymore.
In the long term, only insulation helps. That also saves heating oil and ensures a pleasant indoor climate.
Unfortunately, I am not familiar with blown-in insulation; we have external insulation with mineral wool.
There are certainly subsidies available for insulation.

The stains are all on the north wall or in the corners. These are the living room, office, and bedroom. The heating is permanently on (except in summer). We have set the room temperature to 20 degrees. The doors are actually always open.

This is typical, north is the coldest and also in the corners. 20 degrees in an uninsulated old building is not enough (with insulation it is). Doors should be closed because otherwise the humidity will spread nicely and the water will condense on the coldest wall. The crucial factor is always the ratio of temperature, relative humidity, and temperature of the coldest component in the affected room. You can adjust these variables. And when the doors are closed, it is easier because it is only that one room you have to "monitor."

It is also helpful to always know the temperature and humidity outside, so you can see if airing out is even beneficial.

In that respect, you really can't go wrong. You don’t just need humidity for mold, but also a wall that falls below the dew point. If it’s cold enough for that, the outside air is also so cold that even at 100% humidity the house tends to dry out.

Questions:
How was the house heated in the past? (e.g. central heating with radiators under the windows, stove heating, ...?)
Were double-glazed windows with seals installed?
Was the building mold-free in the past or were there already moisture/mold problems before the renovation?

Questions: How was the house heated in the past? (= e.g. central heating with radiators under the windows, stove heating, ...?) Were double-glazed windows with seals installed? Was the building free of mold in the past or were there moisture/mold problems before the renovation?

The heating is the same, but instead of underfloor heating, it was heated with radiators. We closed off the radiator niches. The windows were also double-glazed with seals, but much older. I think the decisive factor is that the roller shutter boxes above the windows were not insulated. Air could circulate here. We did not see any mold. On the north wall in the living room there was fiberglass wallpaper, and in the bedroom wooden paneling with about 3 cm of Styrofoam underneath. The ceilings were also covered with wood and Styrofoam. I laid out 10 cm of Styrofoam on the floor of the upper floor.

Goods? You mean there was previously a (wenn auch rudomentäre) internal insulation that you have now torn out?