Indeed, there is a problem when a RadDock with nested RadDockZone is added on a page and that page is loaded in RadWindow. As a result a new RadDock cannot be docked into the nested RadDockZone under IE8. The issue is caused by displaying the RadDocks and RadDockZones through iframe when the specified browser is used. The bug is logged and will be brought to the attention to our developers, although I can't provide a firm estimate when a solution is going to be available. You can check the PITS Item by following this link: Public URL
As a sign of gratitude for detecting the problem I have updated your telerik points.
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