Menu/header not behaving correctly on mobile devices

I have a problem with the onetone theme when viewing it on a mobile device (iPhone) in portrait mode:

The menu bar/header on top of the site won’t do its usual behavior of floating at the top of the browser window but it will stay fixed at the very top off the site so you have to scroll all the way up again to click a link in the menu.

You can witness that same behavior on a desktop browser if you make the window width small enough by dragging the corner of the browser window.

When you make the window narrow enough so that this happens another problem is that the header/menu bar suddenly overlaps the slider at the top of the page.

Anybody has an idea how to get rid of that behavior and have the menu bar keep floating even with narrow window width?

This is the site with that problem:
http://www.vongutzeit.com

Many thanks in advance!