Exterior plaster chipped off new building. Moisture problem?

How did you mean working with stones?

On the plaster, there is always a weak point

Plastering down to the ground is always a problem. The execution is technically wrong overall, as no base has been formed and no proper sealing has been carried out. It does not matter how far the plaster has been applied downwards.

Plastering down to the soil is always a problem. The execution is technically incorrect overall because no base has been formed and no proper sealing has been carried out. It does not matter how far the plastering has been extended downwards.

And what is the optimal solution for this?

Since I do not know the structure of your base area, just roughly. In new construction, there is often an insulation board in the ground. Behind the insulation, it should be sealed, and above the insulation board there is sometimes also a plaster mortar. Above this plaster, the sealing should then be extended down into the ground and about 5 cm above the ground level upwards. A base about 30 cm above the soil surface should be formed upwards. This mortar should be suitable. There are, for example, lightweight base plasters, reinforcing mortars for the base, or classically cement plasters. However, this cannot be specified more precisely without details of your base structure. You can find countless execution details from mortar manufacturers on this.