Exhaust guest WC - foul smells are flowing back

hm, if it's not in your horizontal pipe, it must be in the vertical one. Then it has to be above the highest apartment. That means if you install a check valve, the air will take the path of least resistance and you will suck from the neighbors. If it is built that way.

There are also manual check valves, but they reduce the cross-section and increase resistance, so same problem as above. At least it would reduce the suction power. But maybe it's still enough?

You could install your own fan + check valve if you don't mind the hassle.

Or spray foam. Better your own stink than the neighbors'.

Or expanding foam. Better your own stink than that of the neighbors

You are not entirely wrong: The previous occupants had sealed the pipe. Maybe that was the reason. I think I'll try a light flap made of plastic – if the fan then doesn't extract anything at all, another solution will be needed (manual sealing or completely closing). Thanks for the tips!!!! Best regards

My first step would be to go to the property management.