Lightbox Behaving Strange


I don’t know I this is the accurate category…however.

I’ve made an pop-up Lightbox for listing courses. On the main page, if you click on cursussen (courses) it works fine. But if you to and you click on cursussen (courses) it gets messed up. What did I do wrong?

Please help!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)