Dampness in the basement on the floor slab and walls

Hello everyone,
three years ago we bought a house and completely renovated it. Now that everything in the living rooms is finished, I am going down to the basement to make it usable as well, because at the moment it looks more like a junk room.

When clearing away things like paint buckets, furniture, boxes, etc., I immediately felt uneasy. Wherever something had been standing on the floor, there is a damp spot underneath everywhere. Where moving boxes had been placed, the boxes were so soggy from underneath that the floor is cracked and almost disintegrated, and on the walls I saw that on the lowest row of stones the entire plaster is coming off the walls and there is some kind of "mold"? on the walls. When tapping, everything sounded hollow and the plaster fell off. But it only affects the lowest row of stones. Above that everything is solid and also not discolored.

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

I have attached pictures for you.

Now the important question... where does it come from? Rising groundwater? Missing insulation of the floor slab? Missing house sealing at these points towards the adjacent soil?

AND what must/should I do now to get the problem under control?

Additional info: The basement currently has single-glazed basement windows installed and there is no heating in the basement. And it is NOT sandstone.

I hope you can help me.

Thanks and best regards

Explain what was installed ... masonry basement? Waterproof concrete basement? Year the house was built?

It is a house built in 1963. The basement was built like the rest of the house with "normal" stone. Unfortunately, I do not know what the stone is called. A gray one with 3 chambers.

Unfortunately, I cannot say more. Or would there be something else interesting to know to assess the matter?

Dig around the house, reseal the basement wall with bitumen, install ring drainage around the house, check downspouts to see if they are clear! Instead of soil, put loose gravel in the trench around the house, slope away from the house. Then there will be peace for now. Inside, remove the plaster, let it dry, rebuild with cement plaster, paint.

Dig around the house, reseal the basement wall with bitumen, install a perimeter drainage system around the house, check if the downpipes are clear! Instead of soil, put loose gravel in the trench around the house, slope away from the house. Then there will be peace for a while. Inside then remove plaster, let dry, rebuild with cement plaster, paint.

That would also be my assumption .... and solution

I would have done the same and that was my first thought. I just wonder why the walls inside are also damp, basically the walls that do not have contact with the earth outside.

And the floor slab is also marked with damp spots where something heavy stood, e.g. paint cans or full moving boxes.
Or can all of this really come through the outer walls?

Should I actively dry the basement now with a fan heater or just let it dry on its own?