Yes, this is, in a way, causing this error. Please examine the following KB article for detailed information why it occurs, why it is not directly related to the RadControls, but rather to the current scenario, and for a way to work around it: http://www.telerik.com/support/kb/aspnet-ajax/window/cannot-unregister-updatepanel-with-id-updatepanelid-since-it-was-not-registered-with-the-scriptmanager.aspx
. You should either take this RadWIndow out of its manager, or use RadAjaxManager. Please note that I did not explicitly advise you to remove it, it can still be used, you only need to provide accurate controls for its settings (i.e. inside the ContentTemplate of the RadWindow only) - this is explained just below the code sample: "If you are using the RadAjaxManager you should not add the RadWindow as an AJAX initiator or as an updated control. Instead, you should directly use the controls from its ContentTemplate in the AJAX settings.
All the best,
the Telerik team
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