Water on ground slab, cause and leak unknown

From your description of the drainage, I understand that it goes around the whole house. Sewer cleaning companies have cameras with which they can inspect wastewater pipes. Have one of them come and look into the drainage to see if there is water on the relevant side. That surely won't cost more than 200€. Then you can consider that as a possible cause or rule it out.

I'm not an expert, but maybe you could hire a geologist who measures the groundwater level next to the house (maybe even around the house). My feeling would be more towards the groundwater, which can also change.

I will probably do that at some point if I can't find another cause.
Now my architect thinks I should check if rain is coming down through the two chimney pipes, which I can't really imagine because the amounts I soak up every time are considerable. Does anyone have experience with this, can so much rainwater or even condensation water form that the floor slab is washed out by about 2 cm?

Another cause the architect mentioned is that the inspection flap on the sewer is defective and does not open properly, causing the wastewater to back up into the house.

I will probably try to investigate both points over the weekend.
Does anyone have experience with this?

Good luck with finding the cause, at least you have a chance there. However, I see all the effort falling on you, because:

By the way, the shell construction company no longer exists at the moment --> bankrupt.

sounds pretty hopeless. The insolvency administrator is quite a comfortable position compared to people who have claims; they are typically lawyers; and in the case of a profit you must at least accept that the payout will never be 100%. Often it is so much lower that a dispute is not even worth it.

I will probably do that at some point too if I can’t find another cause. Now my architect suggests I check whether rainwater is coming down through the two chimney pipes, which I can hardly imagine since the amounts I absorb each time are considerable. Does anyone have experience with this? Can so much rainwater or condensation form that the floor slab is washed out by about 2 cm?

Another cause the architect mentioned is that the inspection hatch on the sewer is defective and does not open properly, which causes the wastewater to back up into the house.

I will probably try to investigate both points this weekend. Does anyone have experience with this?

You can certainly take a pretty easy look into the chimneys through the sweep openings at the bottom. There should also be traces of leaking water. But I find that very unlikely. Such a chimney has an opening area of maybe 0.05 m² at the top if there is a stove attached... heating rather less. With a few liters of rain per m², hardly anything gets in, and that evaporates again in the rising warm exhaust gas.

Didn’t you try color in the wastewater? That would have revealed the issue with the backflow flap.

How cold are the floor slab or cellar walls? Are they below the dew point? Otherwise, nothing should condense there.

So I’m still guessing groundwater, even if unfortunately that will be the hardest and most expensive option to fix....

You can probably easily look into the chimneys through the cleaning openings at the bottom. You should also find traces of leaking water there. But I find that very unlikely. Such a chimney has an opening area of maybe 0.05m² at the top when a stove is connected.. heating rather less. With a few liters of rain per m² almost nothing gets in and it still evaporates in the rising warm exhaust gases.

Yes, I already checked yesterday, it is dry. In my opinion, that was also too unlikely.

Didn't you try dye in the wastewater? Then the issue with the backwater valve would have been noticed.

I did try dye, but when I look at the amounts of water I pump out, I can also imagine that the dye is so diluted that it is no longer visible.

How cold are the floor slab or basement walls? Are they below the dew point? Otherwise nothing should condense there.

Well, the floor surface is relatively warm due to the underfloor heating. When I looked at the damp spots with a thermal imaging camera, the areas were quite cold, but not below the dew point. What exactly are you aiming at?

So I still guess groundwater, even if unfortunately that will be the hardest and most expensive variant to fix....

Yes, unfortunately I also fear that this is exactly the case.

However, I still want to do a sewer inspection before undertaking any remediation measures, just to avoid unnecessary costs.

If I then have to renovate, I want to possibly proceed step-by-step, i.e. first pressure-inject the wall/joint that is currently visibly affected, and then gradually more until I notice that the water no longer comes through. If possible, I want to avoid breaking out the screed at the spots and perhaps use longer drills to reach the joint and then pressure-inject there, which should be possible according to my expert acquaintance. But that would then be a blind pressure injection.

Earlier I was also advised to possibly make a drilling on the side of the house in the garden to determine the groundwater level.

But I still cannot explain how something like this could happen within a month.
The current tenant living in the basement only moved in at the beginning of February, and at that time the walls were completely dry. There has been neither heavy rain in recent months nor anything else noticeable.

I am at a loss, thanks so far for your answers.
I am still happy to receive ideas.

Yes, I already checked yesterday, it's dry. In my opinion, that was also too unlikely.

Ok, so you can already rule that out.

I did try the color, but when I look at the amounts of water I'm pumping out, I can also imagine that the color is so heavily diluted that it's no longer visible.

Yes, that would of course be possible. That reminds me: a few years ago we had a similar case in a multi-family house where we were renting. After a longer search, chemical analyses of the water were done to find the cause. Based on impurities, one can apparently distinguish sources. It was the desalination system.

So the floor surface is relatively warm due to the underfloor heating. When I looked at the wet spots with a thermal imaging camera, the areas were quite cold but not below the dew point. What exactly are you aiming at?

If something is supposed to condense there, the temperature of a surface that comes into contact with the moist air must be below the dew point. So if all temperatures are above the dew point, nothing should condense there. You could also calculate how much air volume at what temperature difference would be necessary to produce your water entry by condensation. Then you would know if such an entry by condensation is even plausible.

Yes, unfortunately, I also fear that this will be the case.

Although I still want to do a sewer inspection before I take any remediation measures, just to avoid unnecessary costs.

I would do that too.

If I were to remediate then, I might proceed step by step, i.e., first pressure-inject the wall/joint that is currently visibly affected, and then piece by piece more until I notice that no more water is coming in. I want to avoid breaking out the screed in those areas if possible and maybe use longer drill bits to reach the joint and then pressure-inject there, according to my expert acquaintance. But that would of course be a blind injection.

Previously, I was also advised to possibly make a borehole on the side of the house in the garden to determine the groundwater level.

But I still can't explain how something like this could happen within a month.
The current tenant living in the basement moved in only at the beginning of February, at that time the walls were completely dry. There was neither heavy rain in the last months nor anything else unusual.

Such suddenness is of course noticeable. Did the tenant accidentally/unintentionally damage something during drilling work when moving in?

Does the weather in your area provide a reason for rising groundwater? Or are there construction works somewhere that divert groundwater flows or have done so until recently?