This is related to a browser bug in accessing the document.activeElement property inside an iframe under IE9 and IE10. See this KB article for additional information and examples: http://www.telerik.com/support/kb/aspnet-ajax/window/modal-radwindow-in-radwindow-in-ie9-and-ie10.aspx . There are several possible workarounds: 1) remove the Modal feature. 2) Replace the opening logic with the one from this thread ( http://www.telerik.com/community/forums/aspnet-ajax/window/opening-radwindow-from-the-server.aspx ) and add a small timeout before showing the RadWindow. Even 0ms could suffice so the browser has an active element 3) Replace the RadAjaxPanel with an ASP UpdatePanel with UpdateMode set to Conditional 4) Add a function that will provide a focused element and call it in the OnClientBeforeShow event of the RadWindow 5) Add that function and call it in the ResponseScripts of the RadAjaxPanel The function could look like this: function fixIE() { document.documentElement.focus();} where, of course, you can focus some other element on the page. A sample is attached.