Mega menu: Issue with Expand/Slide Menu (isn’t clickable)

I have activated the “Expand/Slide Menu” on my website. Unfortunately I can’t open the page of menu items when there are submenu items underneath. When I click on the item it expands the menu (subsites become visible). I can then click on those subsite menu items and the page opens but I can’t do that for the upper levels of my navigation. How can I fix that?