I cannot get this to work, I'm getting this error:
A server error has occurred while a callBack has being processed on the server.
when I click the button.

<asp:Panel ID="test" runat="server">
</asp:Panel>

...

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ASPxCallbackPanel Panel = new ASPxCallbackPanel();
            Panel.ClientInstanceName = "testpanel";
            Panel.Callback += new CallbackEventHandlerBase(ASPxCallbackPanel1_Callback);

            ASPxButton BrowseButton = new ASPxButton();
            BrowseButton.Text = "ok";
            BrowseButton.AutoPostBack = false;
            BrowseButton.Native = true;
            BrowseButton.EncodeHtml = false;
            BrowseButton.ClientSideEvents.Click = "function(s, e) {" + Panel.ClientInstanceName + ".PerformCallback('test');}";
            test.Controls.Add(Panel);
            test.Controls.Add(BrowseButton);
        }

    }

     protected void ASPxCallbackPanel1_Callback(object source, CallbackEventArgsBase e)
    {
        
        ASPxCallbackPanel panel = (ASPxCallbackPanel)source;
        Label text = new Label();

        text.Text = e.Parameter.ToString();
        panel.Controls.Add(text);
         
    }

If I create the panel and the button in the design for testing it works, like this:

     <dxcp:ASPxCallbackPanel ID="ASPxCallbackPanel1" ClientInstanceName="Panel1" runat="server" Width="200px" OnCallback="ASPxCallbackPanel1_Callback">
    </dxcp:ASPxCallbackPanel>
    <dxe:ASPxButton ID="ASPxButton1" runat="server" Text="ASPxButton" AutoPostBack="false" Native="true" ClientSideEvents-Click="function(s, e) {Panel1.PerformCallback('test');}">
    </dxe:ASPxButton>

So, why doesn't it works codebehind? 

Hi Mattias

In ASP.NET a control hierarchy isn't preserved in the viewstate. You should create it every time(on each page load).
So, you should remove the following condition from your code:

   if (!IsPostBack)

Oh, ok! Yes, it seems to work! :)

But I thought that a ASPxCallbackPanel isn't making a PostBack!?