After a quick search in the net it seems that this is a bug in IE8 with certain sites and/or add-ons, you can see for yourself by examining the following google search: https://www.google.com/search?q=acr_error.htm
Thus, I'd ilke to ask is what happens if you try to load your content page in a simple <iframe> instead of in a RadWindow? If the behavior persists then there is something in this page's code that breaks IE8, as the RadWindow loads external pages in an iframe, as this is the only way to do this.
I would also like to confirm the version of the controls are you using - is it the latest? If not - which one is it? Does upgrading to the newest (currently Q1 2012 with Q1 2012 SP1 coming really soon) resolve this? Please also check what happens if you remove the references to your custom jQuery and use the built-in that comes with the RadControls
as there are also reports in the net that older versions of jQuery (especially if loaded twice) result in this problem.
All the best,
the Telerik team
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