[Solved] ASP.Net Error: Could not find any resources appropriate for the specified culture or the neutral culture

I'm already using ToolkitScriptManager and the rest of the page that uses AjaxControlToolkit works just fine.

I couldn't solve the problem with the AsyncFileUpload control, but as a good side while searching about how to solve this issue, I tried again changing the code and use both FileUpload and UpdatePanel controls.

I changed my code for (instead using AsyncFileUpload) for use FileUpload inside a UpdatePanel, but, here is the catch:

For select and send the file to the server side, the Button that sends the file (once selected with the FileUpload control) must have its own UpdatePanel with its respective Triggers tag.

Example - the tabulation for code samples in this page isn't so good, but, the code added here is properly tabulated: 

In the .aspx:

<asp:UpdatePanel ID="updPnlbtnFileUploadDevols" runat="server">
<ContentTemplate>
    <asp:LinkButton ID="btnLoadClassFileDevoluciones" runat="server" Text="Cargar Archivo" 
        CssClass="chxbx" ForeColor="Teal" OnClick="BtnLoadChargeFileDevoluciones_Click" ValidationGroup="valChrgFileDevoluciones" CausesValidation="true" />
</ContentTemplate>
<Triggers>
    <asp:PostBackTrigger ControlID="btnLoadClassFileDevoluciones" />
</Triggers>
</asp:UpdatePanel>

In the aspx.cs: 

/// <summary>
/// Load file.
/// </summary>
/// <param name="sender">sender</param>
/// <param name="e">e</param>
protected void BtnLoadChargeFileDevoluciones_Click(object sender, EventArgs e)
{
    try
    {
        if (fupld_cargue.HasFile)
        {
            // Here, the file is detected in server-side 
            // and it's been handled as logic demands it.
        }
        else
        {
            // File not detected in server-side.
            MessageBox("Please, select a file");
        }
    }
     catch (Exception handledException)
    {
        // Omitted exception-handling code in this sample.
    }
}

With the previous explanation, I finally could solve the issue I had about sending a file using FileUpload - which was inside an UpdatePanel control.