Crack in the basement ceiling is getting bigger, whom should I contact?

The only major thing that was done was that we redid the screed in the bathroom for the floor with underfloor heating. Otherwise, we laid laminate, ran a few cables, and replaced the windows in the basement. But other than that, not much was changed.

Then best to have it checked by a structural engineer?

Since no professional is responding here, I will give advice as an amateur. Could it possibly not be a cast-in-place concrete ceiling but rather made of concrete slabs laid in sections? Do you have other filler lines at regular intervals? It would of course be unfortunate if a slab joint is exactly at the hole for the staircase exit. However, since I suspect an exterior wall nearby, I would say that the ceiling is securely supported. Why it is still moving after such a long time, I cannot explain.

So is the bathroom where the screed was newly applied over there? Or at least diagonally above it? Then that could be the reason.

I just want to know if the crack is somehow dangerous / could become dangerous or if you can simply fill over it.

I do not consider such cracks dangerous. If you are unsure - ask a structural engineer.

Since no professional is responding here, I will continue as a layperson. Could it possibly be not a cast-in-place concrete ceiling but concrete slabs laid in parts? Do you have more putty lines at regular intervals? It would of course be unfortunate if a slab joint is exactly at the hole of the stairway. But since I suspect an exterior wall nearby, I would say the ceiling is definitely stable. However, I cannot explain why it still shifts after such a long time.

That's exactly the case. There are several slabs, and at regular intervals, there are these putty lines. And yes, right at the stairs, as you can see, there is exactly such a slab joint.

My layman's explanation now is simply: Previously, only one person lived in the house; now four people live here and it is "used" quite differently. Above that is the staircase to the upper floor, which was probably used rather little before and now regularly.

Is the bathroom where the screed was newly applied located over there? Or at least diagonally above? Then that could be the reason.

To the right of the stairs is the bathroom. But that was done last year from March to May.