Hello Martijn,
Hope you’re having a great day!
That’s very odd and I see exactly what you mean.
Do you know if there is any part of your theme that uses that code? Or perhaps another plugin? It does not appear to be coming from The Events Calendar, but rather some other place that is trying to insert itself in the calendar.
If you’re not exactly sure where the code is coming from, you can test by following the steps outlined in this post. It will walk you through the process of deactivated and reactivated your theme and other installed plugins one at a time to see where the code is coming from.
Let’s start there and see what we find.
Thanks!
Geoff