ASP Menu 4.0 AJAX client failed to load

Question

I have a simple ASP.NET 4.0 site (no MVC). Just a Master page containing header, menu and footer. I added the script manager in the Master page too. The menu is an ASP.NET Menu control. Everything is OK on my development machine, but as soon as I move it to the GoDaddy server, the menu stops working. This is a dynamic menu loaded from a sitemap.

My problem is that the first time I load it, it just doesn't work. I always get an 'AJAX client-side framework failed to load error' and the "hover" doesn't work. But as soon as I click on one menu item, it changes the page and everything works fine afterward.

I tried a lot of things that I found here and there on the Internet. I put an EnablePartialRendering="True" to my ScriptManager, I added some code to manage ".axd" html handlers. But none of that seems to work.

Here's the code for the menu (test not final so still messy) :

<asp:Menu ID="mnuGauche" runat="server" DataSourceID="sitemap" BackColor="Transparent" RenderingMode="Table"
                    DynamicHorizontalOffset="0"  Font-Names="Arial Black" Font-Size="small" 
                    ForeColor="#7C6F57" Width="100%" Height="20px"
                    style="margin-left: 0px" Orientation="Horizontal" StaticEnableDefaultPopOutImage="false">
                    <StaticSelectedStyle BackColor="#5D7B9D" ForeColor="White" />
                    <StaticMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" />
                    <DynamicHoverStyle BackColor="#7C6F57" ForeColor="White" />
                    <DynamicMenuStyle CssClass="MenuDynamique" />
                    <DynamicSelectedStyle BackColor="#5D7B9D" ForeColor="White" />
                    <DynamicMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" Font-Size="small" />
                    <StaticHoverStyle BackColor="#7C6F57" ForeColor="White" />
                </asp:Menu>

In my Web.config file, the only thing special that I have is :

<system.webServer>
      <modules runAllManagedModulesForAllRequests="true"/>
    </system.webServer>

Altough there's plenty of stuff on the web, nothing seems to really work for this menu control. Wich is kind of bizarre because it's one from Microsoft themself. Can anyone help me on that one ?