The error message is;

Could not find any resources appropriate for the specified culture or the neutral culture. Make sure “AjaxControlToolkit.Properties.Resources.NET4.resources” was correctly embedded or linked into assembly “AjaxControlToolkit” at compile time, or that all the satellite assemblies required are loadable and fully signed.

This is such an embarrassing error for me to receive. Every time I have gotten it, it has been due to me forgetting one vital control that the Ajax Control Tool Kit requires.

<asp:ScriptManager ID="smManager" runat="server" />

That is right, a scriptmanager control.