Check roof ventilation - suspected incorrect connection

There was no irony at all. It is a factual (and I suspect also the original) cause that many plumbers simply do not have the topic "physics" in their toolbox. Are we actually talking about the same pipe in both threads?

Yes, we are talking about the same pipe. There is only one vent, and the blockages themselves form in the branch of the toilet.
Attached is another picture of the pipes.

Borrow a fog machine from the event technology.

Attached is another picture of the pipes.

... which unfortunately exceeds my understanding - I can neither classify it as a floor plan nor as a section :-(

Yes, we are talking about the same pipe. There is only one vent, and the blockages themselves form in the WC branch.

I already suspected that we are talking about the same pipe. I do not believe in a thromboid blockage, but rather in an unfortunate interaction of air and water columns in the pipe system, mainly in the soil pipe. In the old thread, it is about wastewater that apparently "gets stuck" as a water column on an air column and, after the latter seeps through, sucks in more water from the toilet drain, which then dries out. Presumably, a spiral at the edge of the pipe diameter could give the wastewater a drive not to fall cleanly axially (and/or "disturb" the air column further down by sitting axially before it). Also, a (possibly even perforated) hose - also at the edge of the pipe diameter or hanging movable inside it - could cause a short circuit between the air before and after the wastewater. The exhaust air upwards probably also only "stands" and needs flow stimulation. After a thromboid blockage, one would likely search fruitlessly there, possibly "until one believes in witchcraft."

I think your problem lies literally deeper. The main drainage pipe below the concrete ceiling. There are rules that must be followed regarding when and which branches off of it need additional ventilation. This cannot be determined based on your sketches. It is also possible that there is a section with a reverse slope in the main line; if water stands here constantly, the ventilation in the sewage pipe cannot work completely and clogging is also logical. Normally, your plumber is responsible.

The problem is that both just shrug their shoulders. The piping was installed by our bricklayer. The ventilation was connected by the plumber. Since I don’t know who is at fault or where the problem lies, she also doesn’t know whom I can contact. I have already considered buying a small inspection camera for €90 myself. And then using it to look once inside the pipe.

In the pyrotechnic field, there are smoke pots (different sizes & colors) that would be suitable for this. You can hold them in your hand and they don’t get hot... I use them myself at events, etc.