[NSFW] strange thing happening with submenu items

Hi!

I am building a website for my (Dutch) mother’s art. Everything looks good, thanks for the pretty theme.

There’s a strange thing happening, though. Everything’s looking great on desktop, but when I try to click one of the submenu-items on mobile (so “ACRYL”, “ZWART/WIT” or “KLEUR”), an empty white box opens and covers half the menu. It does seem to take me to the right page, but it doesn’t look good.

What can I do to get rid of the strange floating box? It looks like how the submenu-box looks on desktop.

Thanks in advance!
Romy

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