Fix Error: Only one instance of a ScriptManager can be added to the page
Use a ScriptManagerProxy on your child pages/user controls http://msdn.microsoft.com/en-us/library/system.web.ui.scriptmanagerproxy(v=vs.110).aspx If your code like this <asp:ToolkitScriptManager ID = "ToolkitScriptManager1" runat = "server" > </asp:ToolkitScriptManager> <!-- <asp:ScriptManager ID="ScriptManager1" runat="server"> </asp:ScriptManager>--> Problem Solution ASP.NET ignores HTML comments just like it ignores all tags without a runat="server" on them or that don't start with <% . So, Use below code for comment <%-- < asp : ScriptManager ID = "ScriptManager1" runat = "server" > </ asp : ScriptManager > -- %>