Damp spot on the wall near the front door

Here are some recent photos. As mentioned, the architect said we should remove the paint ...

Before asking about structural measures and liability, one should first find the cause. This is probably not quite easy and I would also consider a building expert.

To me, it looks as if the plaster is absorbing moisture from below. Possibly the sealing of the front door in the base area is not OK?

We also once had such a mystery. IN THE LIVING ROOM of my parents, a few spots of plaster had crumbled in the lower part of the wall. At first, we blamed the exterior wall. But the culprit was a water pipe that ran across the living room on the raw floor and became leaky. The distance to the wall was about 2.5m. On top of it was a dry screed. The water ran under the insulation towards the wall and crept up the plaster. It took several months until we found the error.

What is behind the wall? The guest WC? Then it could be that a pipe is defective there. The water is pressing into the hallway through the tiles or whatever. But that is just a speculation by an amateur.

Hello,
we have just come across your post and have exactly the same problem. The moisture pattern as well as the rooms are identical. We hope that you have found the cause of the problem for your damage and would be happy if you could share your experience with us. For us, basically 2 possibilities come into question: seal on the front door defective or damage in the WC behind it, although no water pipe is suspected in the partition wall. It could still be that the concealed flushing cistern or the pipes at the WC cause the problem, but the moisture only emerges on the hallway side (tiles in the WC). We would appreciate a response.

A floor plan sketch of the premises and the location of sanitary facilities and the location of the pipes would be helpful. What does it look like in front of the front door? Photo? Is there a basement? What type of stone was used for construction?

Rising moisture in the wall carries salts with it. The moisture evaporates on the wall surface and leaves the solids behind on the wall: as efflorescence. Wall drying with electric drying devices does not solve the problem but massively increases it!! Only wall infiltration with wall sealing agents completely dries out the wall. I had various companies in the house and received outrageous proposals costing thousands of €. The membrane solution is only a bad solution. The infiltration funnels are not tight at the wall junction. I drilled into the basement wall myself and dried it out with products from the company in Bobenheim-Roxheim. The company's infiltration containers work on the principle of connected tubes. One treatment for 2 houses cost me 1000 €. No re-tiling, no mess, and everything is tight in the walls.