Toilet flow - something is wrong

Hello. We have been living in the house since 01.06. Since the bathroom upstairs was only finished 4 weeks ago, we have only been using the toilet there regularly since then. For about 2 weeks now, we have occasionally noticed that the toilet upstairs gurgles. This means that so much water flows out of the toilet that the seal with the drain does not fully close and it makes a noise. This only happens briefly because then so much water flows out that the toilet is almost empty. This is not the case with the toilet downstairs, although in my opinion more water could remain in the bowl there as well. Yesterday we experienced the following. The water in the toilets upstairs and downstairs drained somewhat slowly. After flushing, it collected in the bowl and then drained, only slightly delayed. It took about 3 seconds until the water was gone. Since then, a gurgling sound can also be heard from the shower downstairs when flushing. Right after showering, bubbles could be seen coming from the drain there. The water in the shower also drained poorly yesterday. I just cleaned the shower. Result: the water drains perfectly, but the noise when flushing can still be heard. What could be the problem?

Do you have pictures of how the venting of the [Stränge] was carried out?

I also think the ventilation is clogged.

Not directly from the execution. I have attached a wiring plan. In red, I have drawn the pipe route from the WC on the upper floor. The bathroom is located directly above the utility room. At the top right, there is the "D" for shower. The ventilation runs from the bottom right in the utility room through the bathroom over the attic to the outside.

What should be done in case of a blocked ventilation?

Possibly the diameter is chosen incorrectly for the ventilation. Without concrete pictures, it is hard to say. Or it could also be that the line from the shower requires its own ventilation.

The second picture shows the roof, or the connection to the ventilated tile? If yes, then I would definitely say the diameter is chosen too small.

Possibly the diameter is chosen incorrectly for the ventilation. Without concrete pictures, it's hard to say. Or it could also be that the line from the shower needs its own ventilation.

The second picture shows the roof, or the connection to the vent tile? If so, then I would definitely say the diameter was chosen too small.

Yes, the pipe shows the ventilation. According to the tape measure, it should have a diameter of 12 cm.

I have now done the following. Flushed twice and opened the drain at the end of our property. I waited 5 minutes. I estimate the distance to be 20-25 meters. Nothing came through. So it seems to be clogged.

I find it strange why there should be too little water in the toilet in case of a clog.