Hi Jeroen!
Thanks for getting in touch with us! Let me help you with this topic.
That is mostly sure a theme conflict. Please try switching to a default theme, like twenty seventeen, and see if any difference.
We always suggest to setup a dev/staging site so you can test things out and make all plugins, theme and WordPress core updates without disrupting the live site.
Also, please make sure you are using the latest versions of our plugins. Remember you can always download our plugins from your downloads page > https://theeventscalendar.com/my-account/downloads/
Let us know how that goes.
Thanks,
Victor