Hello MrFallen ,
as you wrote me on Gitter with the credentials for your homepage, I fixed the issue. But I guess maybe other people are facing this issue, too, so I decided to answer in this public forum.
To get your menu working on the Offcanvas-position, the menu must be assigned to this position.
So choose your menu and click "Edit".
Now you can assign the menu to other positions
Now it works as expected on mobile devices:
You are welcome Keep on building cool websites using Pagekit