Basement floor - Who knows what these holes are?

I searched again for "holes in the screed" using the image search: it is likely to be a professional screed drying, which of course one should also remember.

However, I do not consider this a lasting defect, as it has been repaired. (3 holes are not much either)
So Hotzenplotz would now have a defective house, even before he moved in.
It would be our house too ;)

Well, then what you are actually saying in reverse is: if I have a wet house and sell it, I simply don’t investigate the reason and then say once it’s sold that I unfortunately don’t know anything.... and I’m off.

It’s not that simple.

when I have a wet house

No, I am saying that if a basement or another part of the house is professionally dried, then it is not damp, let alone wet.

It's not that simple after all.

Exactly!

After Esther now knows where the crop circles in the screed come from, the question remains what she wants to do next with this knowledge. The discussion is drifting towards contesting the purchase application, but is that even up for debate?

Unfortunately no more info from the OP on this......this was just the thought experiment: what if it is measured that it is still wet.

If it is dry she can tile and everything is fine

Let's assume these were drying holes and the damage was completely repaired (the basement appears dry):
1. Should we have been informed about this at the time of purchase?

Since the damage was apparently professionally repaired, only if you were explicitly asked about any past water damage. Then the owner, during whose time the water damage occurred, would have had to inform you; otherwise it is fraudulent misrepresentation. This is common case law of the BGH.