Bubbling in toilet and shower. Does anyone have advice?

good evening,

we bought a condominium 2 years ago. shortly after moving in, we noticed that when the family on the first floor showers or baths, there is bubbling in our toilet, bathtub, or shower, and foam also rises. sometimes the toilet does not flush properly afterward. the bubbling is quite loud and you can smell the detergent the neighbors used. after the pipes were inspected (with a camera), the construction manager said that the family produced too much foam, but the pipes were fine. we also hired an expert, who also thought that it was due to too much foam. but i cannot believe that this is the problem. luckily we have the washing machine in the basement. otherwise, we would not be able to protect ourselves from foam in our apartment anymore. the house is 3 years old. does anyone have any advice?

thank you very much

Funny answer: "Family causes too much foam." I assume that either the cross-section of the drain pipe was planned and executed too small, or the drain pipe has to drain too many connections (apartments) - with an insufficient ventilation cross-section. It is possible that the connections of the individual apartments to the wastewater drain pipe were executed incorrectly. Nothing can be changed about that afterwards - wrongly planned and executed. Breaking open walls/ceilings and improving pipes or connections would be too costly.