Menu disappears

Hi everybody,

I have an issue with the Menu of https://www.toskanaspezial.de/

The Breakpoint ist set to 1240. As intended, the desktop menu appears at 1240. 

But the mobile menu does not appear until 1238. At a screen width of 1239, however, the menu has completely disappeared. Why is that? Can I fix the problem via settings in the Theme Options?

Thanks for your help,

Bettina