AJAX UpdatePanel fails to load in IIS 8.5 (Windows server 2012) - asp.net

Question

Problem Statement
When an event is fired, the updateprogress runs indefinitely and the update panel is not refreshed or the request does not end in IIS 8.5 (Windows Server 2012).
Code for reference below
MasterPage
<%# Master Language="C#" AutoEventWireup="true" CodeBehind="XXXMaster.Master.cs" Inherits="XXXWeb.Common.MasterPage.CoReMQWebSite" %>
<asp:ScriptManager ID="ScriptManagerMain" runat="server" EnablePartialRendering="true" >
</asp:ScriptManager>
<asp:UpdateProgress ID="Updprg" runat="server">
<ProgressTemplate>
<div id="divProgress">
<table width="100%">
<tr>
<td>
<br />
</td>
</tr>
<tr>
<td align="center">
<asp:Label ID="lblProcessing" runat="server" CssClass="FieldLabel" Text="Processing... " />
</td>
</tr>
</table>
</div>
</ProgressTemplate>
</asp:UpdateProgress>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
<script type="text/javascript">
Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(BeginRequestHandler);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
function BeginRequestHandler(sender, args) {
}
function EndRequestHandler(sender, args) {
//debugger;
//if UpdatePanel error occurs...
if (args.get_error() != undefined) {
var Error = "Error Occurred: [Code]:" +
args.get_response().get_statusCode() + " [Message]: ";
alert(Error);
//Hide default ajax error popup
args.set_errorHandled(true);
//...redirect error to your Error Panel on page
//document.getElementById("Label1").innerText = Error;
}
}
</script>
aspx page
---------
<%# Page Language="C#" MasterPageFile="~/XXXMaster.Master" AutoEventWireup="true" ClientIDMode="AutoID" EnableEventValidation="false" CodeBehind="CommonAdminScreen.aspx.cs" Inherits="CoReMQWeb.CommonAdminScreen" Title="Common Admin Screen" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:UpdatePanel ID="upCommonAdmin" runat="server">
<ContentTemplate>
<asp:Button runat="server" ID="btnSaveCommonAdmin" CssClass="btn" onmouseout="this.className='btn'" onmouseover="this.className='btn btnhov'" Text="Save" CausesValidation="false" Width="75px" onclick="btnSaveCommonAdmin_Click" Visible="false"/>
<!-- then there is a grid to be updated on button click -->
Things already tried
Adding handlers in web.config
Web.config
<!--<add assembly="Microsoft.ReportViewer.WebForms, Version=9.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
<add assembly="Microsoft.ReportViewer.Common, Version=9.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>-->
</assemblies>
</compilation>
<!--
The <authentication> section enables configuration
of the security authentication mode used by
ASP.NET to identify an incoming user.
<authentication mode="Windows"/>
-->
<!--<authentication mode="Windows" />-->
<authentication mode="Forms">
<forms loginUrl="logon.ashx" name="AuthCookie" timeout="60" path="/"></forms>
</authentication>
<authorization>
<deny users="?" />
<allow users="*" />
</authorization>
<trust level="Full" originUrl="" />
<!--
The <customErrors> section enables configuration
of what to do if/when an unhandled error occurs
during the execution of a request. Specifically,
it enables developers to configure html error pages
to be displayed in place of a error stack trace.
<customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
<error statusCode="403" redirect="NoAccess.htm" />
<error statusCode="404" redirect="FileNotFound.htm" />
</customErrors>
-->
<customErrors mode="Off" defaultRedirect="ErrorPageInternal.aspx" />
<pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID">
<controls>
<!--<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>-->
<!--<add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>-->
<add tagPrefix="ajaxToolkit" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" />
</controls>
</pages>
<httpHandlers>
<remove verb="*" path="*.asmx" />
<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" validate="false" />
<add path="Reserved.ReportViewerWebControl.axd" verb="*" type="Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" validate="false" />
</httpHandlers>
<httpModules>
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
</httpModules>
The .Net framework is 4.5.2
In a mid of crisis, to make this workable before Friday. Any help/leads will be highly appreciated.
Thanks in advance.