DevExpress CallbackPanel takes forever to load data - asp.net

Question

I have a user control within a child page within a master page in which I need a panel to be displayed according to a check box's checked status.
I could use jQuery to accomplish this, but I need Ajax. Now if you are familiar with DX CallbackPanel you know that this control is just like the ASP.NET update panel.
When I check my check box I call a JavaScript function that calls the CallbackPanel.PerformCallback() function, and this function does what it does.
This is my user control:
<%# Control Language="C#" AutoEventWireup="true" CodeBehind="dx_auto_saxs_sales.ascx.cs" Inherits="bime.shokri.v1._0._1.Forms.dx_auto_saxs_sales" %>
<%# Register Assembly="DevExpress.Web.v15.2, Version=15.2.10.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" Namespace="DevExpress.Web" TagPrefix="dx" %>
<script type="text/javascript">
var CallbackInQue = false;
function CheckBoxBimeSalQabl_CheckedChanged(s,e)
{
if (ASPxCallbackPanel.InCallback())
CallbackInQue = true;
else
ASPxCallbackPanel.PerformCallback();
}
function OnEndCallback(s,e)
{
if(CallbackInQue)
{
ASPxCallbackPanel.PerformCallback();
CallbackInQue = false;
}
}
</script>
<dx:ASPxCheckBox ID="ASPxCheckBox2" runat="server" Theme="MetropolisBlue" Text="بیمه نامه سال قبل دارد" Font-Names="yekan" CheckState="Unchecked" ClientInstanceName="ChechBoxBimeSalQabl">
<ClientSideEvents CheckedChanged="CheckBoxBimeSalQabl_CheckedChanged" />
</dx:ASPxCheckBox>
<dx:ASPxCallbackPanel ID="ASPxCallbackPanel" runat="server" ClientInstanceName="ASPxCallbackPanel">
<SettingsLoadingPanel Text="صبر کنید&hellip;" />
<ClientSideEvents EndCallback="OnEndCallback" />
<PanelCollection>
<dx:PanelContent runat="server">
<asp:Panel runat="server" ID="pnl_formerBimeName">
<div class="three fields">
<div class="field">
<label>تعداد سال های تخفیف <span style="color: #c66050">●</span></label>
<dx:ASPxComboBox ID="ASPxComboBox7" runat="server" ValueType="System.String" BackColor="#efefef" CssClass="round-corner-control" Theme="MetropolisBlue" Font-Names="yekan" NullText="انتخاب کنید" Width="90%">
<Items>
<dx:ListEditItem Text="1 سال عدم خسارت" Value="0" />
</Items>
</dx:ASPxComboBox>
</div>
</div>
</asp:Panel>
</dx:PanelContent>
</PanelCollection>
</dx:ASPxCallbackPanel>
The purpose is to display Panel(pnl_formerBimeName) when CheckBox is checked.
Now this is my Page_Load(sender,object) method which is called asynchronous when Checkbox is checked:
protected void Page_Load(object sender, EventArgs e)
{
pnl_formerBimeName.Visible = ASPxCheckBox2.Checked;
}
Everything is OK until I deploy my website to my server. After that when I check the checkbox the "Loading..." the text appears and takes forever to complete. I really don't know why.
Just in case you wonder about this problem in action, I give you the mentioned page here.