Blockage below the floor slab - puzzle

Hello everyone, we have been living in our house for 9 months. We now have the 3rd clog beneath the floor slab. The pipe runs under the foundation across the yard to the drain. From the drain to the house, no damage or clog is visible.

There is gurgling throughout the entire house and all drains flow very slowly or not at all (also upstairs), except for the kitchen. We notice the clog because the siphon of the upstairs toilet empties itself. So far, we were able to remove everything with plenty of water using home remedies. But now there seems to be something seriously wrong.

Now the puzzle:

When we run water in the kitchen, it arrives cleanly at the drain. Everything else does not reach it. When we go with a camera into the drain in the utility room (the camera from the friend is too big for anything else), the camera is full of water after 2 meters. However, the kitchen is located behind the utility room connection when viewed from the drain. How is this to be explained? What could be the fault?

Picture:
K Kitchen
H Utility room
E Ventilation, the bathtub is also connected there

The guest bathroom connects to the pipe from the left.

Is there a wastewater planning for the building application?

This does not help you, but I always warn against such installation under the slab.

How many of the lines go to the roof with ventilation? Are the information that appears, for example, with the search term "Normgerecht entwässern IKZ," already known?

Where is the bathroom upstairs connected, the one where the siphon is being emptied?

That should be where the grounding stands up. I believe the shower on the upper floor, as well as the washbasin, go down the pipe at the vent. Only the toilet goes down there.

There is only one vent.

I have just read the requirements according to [Norm].

Show this to a competent plumber. I think the ventilation is insufficient, possibly in combination with an inadequate slope under the floor slab.