Scrollbox distorted in Internet Explorer

On older versions of IE, 8 and 9 specifically, the scrollbox appears stretched across the bottom of the window and the close button is barely visible to the left of the window. Here’s a screenshot:

https://www.dropbox.com/s/prnk109fjw13sfb/mailmunch.jpg?dl=0

website: http://www.driscollchildrens.org

As much as I’d love to ignore IE8 & 9, it’s widely used to view our site.

Any ideas how to fix this?

https://ww.wp.xz.cn/plugins/mailchimp-forms-by-mailmunch/