Hi Thorsten,

I am sorry for the late reply.
I have inspected your site and it seems like it is coming from your current template. When I commented out this line: http://screencast.com/t/UAM2koLwErY , the button is showing correctly. You can try use this css override code:

.nav-collapse,

.nav-collapse.collapse {

	overflow: inherit;

	height: 0;

}

If this issue is still persist, perhaps you can consult this issue with your template provider.
Please do update us back on this issue.