Significant water damage in the new building - but caused by what?

Thanks first of all for your answers!

What do the basement walls look like? Also WU concrete?

Are there photos of the finished (poured) basement from the outside?

After the soil samples showed that there is no penetrating water, a masonry basement was chosen. The reason was that due to the basement apartment (commercial area) there is a lot of flush-mounted electrical installation in the exterior walls, which is somewhat more difficult in WU concrete.

On the floor slab, a strip of tar paper(?) was laid under each wall. In addition, after the first row of stones ("kimmschicht"?), a sealing slurry was applied as a barrier - this seems to work well.
From the outside, the entire basement was then painted twice with black coating and finally sealed with tar paper. It was welded up to the bottom edge of the floor slab. In the soil, EPS boards were then used for insulation.

The previous speakers have, of course, raised another issue regarding the sealing of Bopa to the wall. In short, you have no joint tape/sheet metal and therefore no protection against accumulating or even pressing water. Now, we all do not know the exact circumstances and can therefore only give you abstract advice. Due to the damage that has already occurred and its extent, I can only advise you to immediately consult an expert who compares the construction method with the soil report and analyzes any execution errors and/or conducts further investigation of the causes.

If there was a soil report?!!? that concluded that no groundwater or accumulating water was to be expected, then what happened here?

It is quite possible that the contractor then relies on the fact that no accumulating water or even groundwater was to be expected, making it difficult to hold him accountable. In that case, the author of the report would rather be the one to be scrutinized. But all of this is just speculation from afar!!!

You made the “mistake” of still building a basement these days. I put mistakes in quotation marks because as a layperson, one quickly gets persuaded or relies on the statements and decisions of the professionals. I would never drive a car without a seatbelt and build a basement without a waterproof shell, whether I am building in a moor or on the top of a hill.

Do not rely on your elemental insurance either, it does not cover rising water, only surface water (flash flood, heavy rain, etc.).

I’m keeping my fingers crossed for you and please keep us updated here on how this matter progresses.

You live in NW, isn't there also flooding due to heavy rain? Sometimes groundwater rises higher than expected there too.

Well, that's unfortunate now. If the tar layer was damaged somewhere after filling, it could press through the walls beyond the pipes. A specialist will really be the best solution. Maybe talk to the building insurance to see what can be done?

Even if the report states that there is no groundwater, the design water level for impermeable soils must be set at the height of the ground surface (GOK).
Impermeable soils have a Kf value of < 1x 10^-4 m/s.
So if there is no nice sand or gravel, then the soil is not permeable. This looks like clay here... through the basement excavation, a basin has been created that fills up first during heavy rain and then creates pressing water.
Therefore, be sure to have the soil report checked.
You can also upload parts of it (layer directories and soil properties, building waterproofing).

No additional drainage was installed, right?

And already inform the building insurance as a precaution/parallel.