I went to Toolset > Custom Fields to edit the custom field group containing the "Ik wil het aanbod gratis bekijken" field to verify what value is saved to the database when checked, to use that value in the condition for the conditional display group in the form, but what I found was problematic.
The field "Ik wil het aanbod gratis bekijken" isn't a checkbox field (singular), but a checkboxes field, which doesn't actually have any options defined (see screenshot).
It seems to me that a field "Ik wil het aanbod gratis bekijken" should be a checkbox field, and trying to use a checkboxes field for the display condition may prove problematic.
You cannot convert a checkboxes field to a checkbox field.
I suggest you create a new field which is a checkbox field and add that to your form and create the display condition using that field.
Remove the current checkboxes field from the form and the custom field group.
I would recommend being sure to use a different slug for the new field than the current checkboxes field to avoid any potential issues with previously saved data.
Once you have done so you can test whether the conditional display works when clicking the checkbox in the form.
I removed the problem checkboxes field and replaced it with a checkbox field with the same name (but the slug has an underscore added at the end to distinguish it from the checkboxes field), and set the condition to check whether that new checkbox field is checked.
On the form I noticed that introduced some styling issues for the checkbox, so I added a couple of CSS declarations to the form to fix those.
Please go ahead and check the form to confirm it is working as expected.