Ok, I’ve found the problem meanwhile, but it’s a kinda strange behavior:

We are using Enfold and the Slider Revolution plugin. So in the avia editor we can add revsliders as content element.
If we add a revslider to a page, not only the code for the slider itself is added to the page output in the frontend, but also are several avia classes added to a lot of other content elements within the whole page.
I don’t know why that happens. Is that any problem or did you really program it like that??

Obviously our home page / our child child theme (that was not developed by me…) needs a revslider to output all code correctly. If we remove the revslider, some classes and therefore style definitions are missing, so other content elements in the page don’t look as they should any more and responsive design for multicolumns doesn’t work any more.

Stef