Thanks for your answer. I checked this and the WPtouch theme was assigned to our web-menu. I created a new one for the mobile version. There I added the HOME site as the first entry. So the editor has the following topics:
HOME
Profil
Campus
Unterricht
The first entry – in this case HOME – is not shown in the menu of the mobile version. For me this workaround is OK, but I am not sure if this is the correct behaviour.
