5-6 cm wide static concrete wall in the outdoor area

Whether it tips over or not is irrelevant. The neighbor must remedy the encroachment on your property.

I see it the same way. However, the neighbor doubts that the wall will continue to tilt. A written expert opinion has already been requested, but so far nothing has been received.

I think you have to pursue two tracks here. On the one hand, have the neighbor, through a lawyer, compelled to remove the wall from your property and to erect a proper support. And on the other hand, have an expert take a look to ensure that it is ultimately done properly. What exactly to look for, I don’t know either. Possibly a geotechnical engineer? Experts in civil engineering or landscaping?

What exactly one is looking for there, I don't know either. Possibly a geotechnician? Expert for civil engineering or horticulture?

A structural engineer can also recalculate that.

A structural engineer can also recalculate that.

With finished L stones, you don't need a structural engineer. They have been around for many years and hold up if the ground is suitable.

Whether it falls over or not is not relevant. The neighbor has to remove the encroachment on your property.

As I understood it, the bend / base of the L stone is set back from the boundary and only the upper part is now on the boundary because it tilted or was installed crookedly.

Correct . The wall is a few centimeters off and leans over. The weakest point of the L-shaped angles is probably the vertical wall and the bend (corner). The wall is about 50 years old. If I now want to attach a concrete block (vertically down) there, I suspect that the ground beneath the vertical wall will be loosened. But that also means that the wall could become even more unstable. Is there a flaw in my theory?

The wall in the ground is probably only meant to hold back moisture at 5-7 cm. The frost will probably pass through, right? How should the wall theoretically be constructed in the ground? It all seems a bit strange to me.