Sys.WebForms is not defined while Sys is defined

You have to set the ScriptManager property "SupportsPartialRendering" explicit to "true".
(Additionally to EnablePartialRendering="true")

<asp:ScriptManager  id="ScriptManager" runat="server" SupportsPartialRendering="true"  EnablePartialRendering="true"/>

This will include  MicrosoftAjaxWebForms js