Hi there, Merijn!
Ah, I see what you’re saying. Clicking the links simply takes you to the top of the page. That’s no good!
Right off the bat, I do see some errors in the console when I view your site. Cleaning some of those up might have an effect here.
I also see that you noted having tried activating a default WordPress theme (like Twenty Fifteen) and deactivating all other plugins. Just out of curiosity, did you see any difference in those conditions? How about activating Twenty Fifteen and deactivating all other plugins at the same time?
Let’s start there and see what we find. 🙂
Cheers!
Geoff