Dampness in the basement on the floor slab and walls

I am now going down to the basement to make it usable as well,

To coordinate measures, it is important to know what you plan to do with the basement. You should be clear that with reasonable means, it can only become a dry storage room, not a living space, no matter what ceiling height is present there.

Where something stood on the floor there is a damp spot underneath everywhere.

The sealing of the floor slab was rather poor in the 50s and 60s, and capillary breaking layers were rarely installed underneath. Ground moisture simply rises from below through the floor slab, probably through a former simple bitumen coating. A classic test for this is to leave a newspaper covered with plastic foil lying for a few days – but you don’t need that anymore.

In the lowest row of stones the entire plaster is coming off the walls and there is some kind of "mold"? on the walls.

The moisture rising from below only reaches up to the horizontal barrier. This was (and still is) installed above the first row of stones. Back then mostly 600g bitumen felt. This still seems intact, which is good and important. The moisture rises only up to this point and does not continue further up the wall. The "mold" is salt efflorescence, often mistakenly called saltpeter efflorescence (unless you regularly pee against those walls or keep cows there). The ground moisture penetrates the floor slab and masonry, and especially in the masonry it evaporates on the surface in the basement. The salts washed out by the moisture remain as crystals on the concrete surface. The plaster flakes off for the same reason. The water evaporates and dissolves the plaster. This is further intensified by additional usually waterproof coatings applied from the inside.

It is a house built in 1963. The basement would be built like the rest of the house with "normal" stone.

Even in the 60s, clay- or loam-containing stones were very rarely used as basement masonry. Usually, lime sandstone or concrete was used, concrete often also as stone, not only cast in place. Back then it was obtained cheaply from remnants of war ruins – rubble aggregate concrete. The color varies depending on origin.

It is both on the exterior walls that lie in the ground and on the interior walls and also not in all basement rooms but only in 2 out of 4 rooms. The others are dry both on walls and floor.

Interior or exterior wall does not matter since the water comes from below and not horizontally from the side. Your horizontal waterproofing, back then usually just a simple bitumen coating, therefore still seems intact or the horizontal moisture ingress is not too high. With permeable soil, undamaged rainwater pipes, and terrain sloping away from the house, the waterproofing does not need to do much more either. That the basement is only partially damp can be due to the surrounding soil, which is partially permeable material also at the bottom. If it is the west side that is damp, it is simply because the precipitation there is stronger and the soil below tends to be moist, or other reasons. You certainly cannot say this from a distance.

Digging around the house, resealing the basement wall with bitumen

This would probably cost between 15 and 25k euros through external companies, destroy the entire garden, and have exactly zero effect. But from a distance, with a house without a basement and without having to pay for the work, such things are quickly guessed.

The moisture rising from the ground can be relatively well controlled with your intact horizontal barrier.
First remove all loose plaster and then smooth it again with waterproof, not waterproofing, concrete repair mortar (e.g. Lugato R&R) or renovation plaster. At the transition to the floor slab, create a concave fillet out of the mortar so that the subsequently applied waterproofing does not have a hard edge there. Adhesion is simply poor on 90-degree angles. Then clean everything nicely (dust, dirt). Then apply waterproofing from the inside. Depending on the load case (hard to say from a distance), a flexible mineral waterproof slurry is sufficient; possibly more is required. What is definitely sufficient (seals virtually all load cases from all directions) is a bitumen-free 2-component hybrid waterproof slurry applied in two layers. Apply this on the entire floor and up the walls above the horizontal barrier. As said, the concave fillet is helpful. Alternatively, install a flexible sealing tape instead of the fillet (it does not have to be the exorbitantly expensive 240 mm wide one; the 12 cm wide one from the sanitary sealing area also works very well on the inside). Follow the manufacturer's instructions during application (wetting, processing and drying time).
The waterproofing can then be left as it is; it is permanently solid after drying. But it can also be tiled. I would advise against all other floor coverings.

What this does not prevent is condensation on walls and windows from the inside due to cold walls. This can be controlled with proper ventilation (open windows when it is colder outside than in the basement).
Digging up and large-scale insulation from the outside help here too, but the cost-benefit ratio in your case is definitely not given.

What Till writes sounds plausible. I would try it like that for now. You can always dig it up later. K.

Wow, thank you very much for the detailed explanation and coherent presentation.

I only have two questions left now.
If I have knocked off the loose plaster from the walls, should I first let the room or the damp masonry dry, or can I already start repairing with the mortar right away?

And I just looked up the sealing slurry and there are also two different variants, where I can't see any difference from the product description. There is one that is applied to the cellar, and one that is a bit more liquid for brushing.
What is the difference there? Unfortunately, I can't tell when I compare the two products directly from the description.

When I have knocked off the loose plaster from the walls, should I first let the room or the damp masonry dry, or can I start repairing directly with the repair mortar?

Nothing will dry there because moisture is coming from below. Repair mortar based on cement doesn't care about that. You also don't currently need a dehumidifier for room air. It doesn't help as long as the source isn't eliminated. After sealing, you can run a dehumidifier in the summer months if the basement gets too damp, but right now that's not helpful. The mortar’s data sheet also lists drying times until the next layer can be applied. You should definitely observe those.

There is one that is applied with a trowel in the basement and a somewhat more liquid one for brushing. What is the difference?

Most sealing slurries can be applied by brushing or rolling as well as with a trowel. They are just mixed with more or less water. The method depends on the stress case, but for negative waterproofing, like here, application by trowel in two layers is usually recommended. Minimum (and maximum) layer thicknesses can be found in the respective technical data sheets. You can find these sheets online in most shops and always on the manufacturers’ websites. There you can also calculate how much material you need. For the application, it is advisable to use a layer thickness comb to be sure. This avoids areas that are too thin where moisture can penetrate again. As already mentioned, with a two-layer application of bitumen-free hybrid sealing, you cover all possible stress cases, but in your case, the FLEXIBLE (!) sealing slurry should be sufficient.

Oh, and if the processing instructions say: "The substrate must be dry, dust- and dirt-free" – that is not always to be taken literally. Mineral substrates should always be pre-wetted/moistened before applying plaster, mortar, or seal coatings so that they are surface damp. This binds the dust that is always present in basements; otherwise, nothing will adhere!

Thank you very much.
I will then promptly address the interior and get back here with any questions or generally provide feedback on how it worked.

Outside, a digger will very likely work reasonably promptly, so it can clear the two walls lying in the ground so that I can apply the appropriate sealing and insulation there.

Thank you for the help and detailed explanation.

dertill said everything perfectly. I would just add: the existing horizontal barrier must not be damaged during any of the work now. (especially when removing the old plaster) Your internal sealing (as described above) must then be seamlessly connected there.

PS. and don’t let anyone convince you to line the basement with a thin WU concrete layer. Unfortunately, this is still often done in basement renovations, but it doesn’t work because WU concrete is only watertight from a certain layer thickness, which you usually don’t have in the basement. Thin WU concrete is useless.