The usage of the RadScriptManager and the standard ScriptManager on one and the same page is indeed not possible since there could be only one script manager per page. As for your current problem, when the content of the RadWindow is defined via its NavigateUrl property, this content is loaded in an iframe element via a standard GET request which has nothing to do with the current page. Since the Report Viewer requires a ScriptManager to serve its resources, such should exists on the page. In case you are experiencing problem with the Report Viewer itself, please use the corresponding forum.
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