Do you get this problem on a laptop or a PC with old videocard? I am asking this because such problem usually occurs when the page is displayed on machines where the card is either old or the video drivers are outdated. The reason for the problem is that for the modal overlay in IE, we are using CSS filter which is a DirectX object. If there is a lot of content on the page, the browser might not be able to properly show the filter applied over the modal overlay and will make it solid instead of semi-transparent. Try updating your drivers / DirectX version and see if it helps.
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