If you have added NavigationControl from the Toolbox this control explicitly "adds" ScriptManager on a page , because it has an attribute - [RequireScriptManager(true)]. If you have dropped the RadMenu control you have to use ScriptManager. By default we add the ScriptManager internally inside the RouteHandler which is a base class in Sitefinity that handles all requests.
We made some changes to the way that ScriptManager is added to a page and fixed some issues that will be available in the weekly internal build.
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