Modal's backdrop dissappear on scroll

I'm working on a website that shows a lot of products. When the user clicks a product, the product information is opened in a modal, via AJAX. To perform this task I'm using Saribe's eModal. Everything works great, but when I have a very big modal and the user has to scroll, I have 2 problems: 1) The backdrop dissapears. 2) In InternetExplorer, the usar can't scroll with the navbar.

This is the css of my backdrop.

background-color: #000;
width: 100%;

Something really strange is that when I open the Chrome DevTools, the problem is solved!

You can check the website here. Do you have any idea of what the problem may be? I've tried with height attributes and lot of things, but anything seems to work.

Thanks!