Menu display issue with Vantage theme

Hi,

Having problems with a responsive menu for my forum using the Vantage theme. It seems to be getting confused when determining whether in desktop or mobile mode and ends up displaying both on my desktop. Also, on my desktop and on my mobile, the following is also displayed:
< class="menu-mobilenav-container">

I’ve deactivated all my plugins and the issue remains. I’ve also switched themes and the problem disappears. Could someone (from SiteOrigin?) please have a look and check it out?

Many thanks,
Neil

The page I need help with: [log in to see the link]