modal popup extender is not working in visual studio 2013 for web

Question

I am encountering this issue:

asp.net ajax client-side framework failed to load

when I tried to open the modal popup-blocker.

How can I solve this? I've been struggling for weeks...

Here's the code for the same:

<form id="form1" runat="server">
    <cc:ToolkitScriptManager ID="sc1" runat="server"></cc:ToolkitScriptManager>
<div>

    <asp:Button ID="ModalCall" runat="server" OnClick="ModalCall_Click" Text="ModalCall" />

    <cc:ModalPopupExtender ID="mod1" runat="server" CancelControlID="btn1" PopupControlID="pan1" TargetControlID="ModalCall"></cc:ModalPopupExtender>

    <asp:Panel ID="pan1" runat="server">

        <asp:Button ID="Register" runat="server" OnClick="Register_Click" Text="Register" />
    <asp:TextBox ID="Email" runat="server"></asp:TextBox>
    <asp:TextBox ID="Password" runat="server"></asp:TextBox>
    <asp:TextBox ID="ConfirmPassword" runat="server"></asp:TextBox>

    <br />
    <asp:Button ID="Login" runat="server" OnClick="Login_Click" Text="Login" />
    <asp:TextBox ID="LoginEmail" runat="server"></asp:TextBox>
    <asp:TextBox ID="LoginPassword" runat="server"></asp:TextBox>
    <br />
    <asp:Button ID="CallService" runat="server" OnClick="CallService_Click" Text="Call Service" />
    <br />
    <asp:Button ID="ChangePassword" runat="server" OnClick="ChangePassword_Click" Text="ChangePassword" />
    <asp:TextBox ID="OldPassword" runat="server"></asp:TextBox>
    <asp:TextBox ID="NewPassword" runat="server"></asp:TextBox>
    <asp:TextBox ID="ConfirmChangedPassword" runat="server"></asp:TextBox>

    <br />

        <br />
        <br />
        <asp:Button ID="btn1" runat="server" Text="hide" />
    </asp:Panel>

</div>
</form>