Menu appears only for a short moment

Hi there,

I’ve a problem with a page. When I click on the menu (in small resolution e.g. smartphone) the menu appears only for a short while, right after it disappears. This makes it impossible to click on another menu point. It happens only on a single page, where I use the “Portfolio Filter Gallery” [ redundant link removed ].
Does anybody have an idea what the problem could be? I’d be very thankful if someone could help me.

• This topic was modified 6 years, 3 months ago by Jan Dembowski. Reason: Moved to Fixing WordPress, this is not a Developing with WordPress topic

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