Theme Conflict

Hi,
I am working on a clients-site which really old. It is based on XHTML. My job is to make the design responsive. Therefore I tried to use your phantastic plugin, like I alsways do with responsive sites. Usually it works, but in that case not. If you press the menu-button, nothing happens.

Do you have any idea where to search for the compatibility problem? Is it because of XHTML? I did not work with XHTML since years, but I suppose it should not be problematic. The design is rather simple so recoding would also be an option if I cannot dedect the mistake in a reasonable amount of time. But maybe you have an idea.

The URL: https://homepage-4-you.net/metallschilder24/
The theme: BLANK-Theme

Thsnk yoju!
Raphael