This iframe element has an empty html tag that can cause issues with the Section 508 validation.
Also, the iframe is not used so it does not need to be present in the markup at all.

The following script added to the OnClientShow event of the RadWindowManager can remove this iframe:
		function OnClientShow(sender, args)
		{
			if (!sender._isPredefined) return; //check if it is a predefined dialog
			var frame = $telerik.$("iframe", sender.get_popupElement());
			frame.remove();
		}