The depicted spots can be seen on all levels between the floors, but only in the stairwell, where no solid wall is bearing (e.g. transition ground floor / 1st floor, transition 1st floor / 2nd floor). It is an open stairwell with a staircase from the basement to the 2nd floor. The builder said at the time that it could not be done otherwise, as different materials meet here. Therefore, I have now decided to cut open at one spot before the warranty expires. To be honest, I really cannot see different materials.
In other places we do not have such transitions, where the gypsum block walls are only built between floor and ceiling. So far, we actually had the problem in another room that the plaster detached behind the wallpaper. There was probably about a 1 cm thick layer on it. The detached plaster was removed, replastered. All tiled wallpapers taken down, then new ones applied and everything repainted afterwards. (under warranty, of course).