Upon closer examination: you're right. Because the menu has a roll-out effect, I thought it was a JS-menu. But with JS off, it still works and shows the same problem. Only the roll-out effect is then no more.
Go with SB65's advice, Laura.
How to: Target IE in, Position in, Center in, Create a Fixed ('Sticky') Footer with, and Create a Drop-Down/Fly-Out Menu with CSS: Website Laten Maken Amsterdam