The siphon or shower drain empties when flushing the toilet.

Has anyone already said "fehlende Entlüftung"?

Have two assumptions:

1. Incorrect or missing ventilation
2. Someone produces such large piles during toilet use that the entire pipe system collapses in on itself.

Has anyone already said "missing vent"?

:) So the ventilation outlet itself is connected to the ventilation tile. That is not a problem. And I assume that the wastewater pipe from the guest bathroom ends in the main wastewater pipe. Therefore, there should actually be a connection to the vent.

2. Someone produces such large piles during toilet visits that the entire pipe system collapses in on itself.

How big is an average pile that clogs a pipe system? :)

From that point of view, there should actually be a connection to the vent.

Yes, but the vent works as an entire system. If, for example, negative pressure occurs somewhere (also e.g. low-pressure outside air), that can have an impact on it. You need to have the respective air volumes checked to see if they are properly designed. If it were a "normal" floor drain, I would say you could just pour water in there because it might have dried out. You alone wouldn’t be able to manage that.

I assume the flat shower channel siphon has already been vacuumed out before the toilet wastewater even reaches the main line. Channel <=> WC are only about 1.20 cm apart.

I assume the flat shower channel siphon is already vacuumed before the toilet wastewater even reaches the main pipe. Channel <=> toilet are only about 1.20cm apart.

Yes, that's what I also think.