FancyBox image loading trigger error

On this site when you click the house icon, it triggers a fancybox popup inside an iframe from a cross domain. After further investigation, it turns out the slideshow is inside the iframe, which is inside the fancybox:

$a_link = 'onclick="parent.postMessage(''.$my_URL.'', '*');"'; 

But after a couple seconds (I guess when the second slide loads) this error appears:

Forbidden You don't have permission to access

Why does this happen? This used to work until the company changed the slider. Is the iframe code triggering the error? The fancybox? The jQuery?

Here is the inspector error:

GET http://mesvideos.ca/previsite/%7B%22width%22:300,%22height%22:600,%22startT…ss%22:%22ads-box-inner%22}],%22class%22:%22ads-box%22},%22dummyId%22:true} 500 (Internal Server Error)