I've been setting up a pop-up which previews correctly

Hi, I've been setting up a pop-up which previews correctly (except for a picture I am trying to add). However, when somebody uses the pop-up he is redirected to 404 page. I noticed that Infinite SEO shows an URL related to this popup which runs like http://www.thewritemind.be/nieuwsbrief (see screenshot) - but this URL is non-existing. How to solve this & get the pop-up working correctly?

Luc