Strange horizontal fold in the wallpaper

I have now removed the wallpaper (at least partially) at the relevant spots. The wallpaper could not be removed quickly, so I only looked underneath at various points and did not remove an entire strip.

There is indeed a relatively small continuous crack under the fold of the wallpaper.

I then lightly scraped the plaster at the crack with a putty knife to see how deep it goes. It appears that it only affects the top layer of the plaster. As soon as I scraped off a little, it was no longer visible.

Is it correct that at the moment nothing indicates a serious problem? Do you share the same assessment?

What I do not understand, however, is:

1. Why is the crack at the same height everywhere? What could be the structural reason for this?
2. Why is the crack so extensive --> multiple rooms and over several meters.
3. Can I take any preventive measures, since there might be a structural problem after all, or is it better to wait for now?

Thank you very much.

As soon as I have scratched off a little bit, it is no longer visible.

Did you remove the dust after/during scratching as well? If not, it could be that you are scratching the plaster into the crack. Then it looks as if the crack does not continue into the deeper layers.

Is it correct that at the moment nothing indicates a serious problem? Do you come to the same assessment?

Yes. Something has "settled" there. Who knows exactly what was/is at this height (maybe started bricklaying again after a break?). But these are cracks like the ones you repeatedly find in new buildings (or in old buildings hidden behind wallpapers).
Preventive measures? Apart from looking for another house, there are none...