Such an error usually means that the script is executed too early. This often happens if you are opening a RadWindow from the server code without using the Sys.Application.Load event, or if the partial postbacks include the RadWindow/RadWindowManager. I advise that you examine the following sticky thread on the subject: http://www.telerik.com/community/forums/aspnet-ajax/window/opening-radwindow-from-the-server.aspx
Also, if your version is old (i.e., released prior to the IE10 release), it is possible, that it may have issues with that browser. In this case you should update to the latest. The second alternative would be to try an X-UA Compatible meta tag to force IE10 in IE8/IE9 mode, but I cannot guarantee this will work.
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