I found out that the problem lies in a change which I got from a X support person a couple of weeks ago. They told me I should insert into the Customizer CSS the following:
.x-navbar-fixed-left {
top: 0px !important;
left: -230px;
}
With this change the navbar appears correctly in non-mobile view, but incorrectly, as shown, in mobile view. When you remove the left-tag or set it to 0 the navbar overlays the left part of the layout on non-mobile view (therefore the change was implemented), but it appears correctly in mobile view. So when I got this change the X support stuff did not look what happened with the mobile view, the same did I. 
The problem now is that this change happened a couple of weeks ago. And people with smartphones could not access the site since then.
So I really need a fast solution to this. Hopefully you find it for me, because I know only a little about all this programming stuff.
Albert