Although I'm not sure this was the cause (it seems there should be a different error for this), I made the following correction in the downloaded code and now it works:
Original markup in Frontend.Master:
Code:
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
<Scripts>
<asp:ScriptReference Name="jquery" />
</Scripts>
</asp:ScriptManager>
Corrected:
Code:
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
<Scripts>
<asp:ScriptReference Path="~/Scripts/jquery-2.1.1.min.js" />
</Scripts>
</asp:ScriptManager>