Hi,
Apparently you have the older version of our VM Template. That issue happens when any group is blank. (for instance, this group: Nieuwste artikelen, in your website, doesn't have any products. If you unpublished it from VM Backend, the problem will be gone and slider will start working..)
But that issue has been fixed and not happening right now even though any group is blank. I have checked your order status, you are eligible to receive free updates. The new version of our VM Template has been sent to your email address.
Cheers.