SharePoint .net CSOM getting "The remote server returned an error: (403) Forbidden"

Hi,

We are getting "The remote server returned an error: (403) Forbidden"  suddenly in production environment while using .Net CSOM. We tried to read the production site title with the below code, still facing same issue.

staticvoid Main(string[] args)

        {

            ServicePointManager.ServerCertificateValidationCallback =delegate { returntrue; };

           ClientContext clientContext = newClientContext(@"https://sitename...");

           Web site = clientContext.Web;

            clientContext.Load(site);

            clientContext.ExecuteQuery();

           Console.WriteLine("Title: {0}", site.Title);

           Console.ReadLine();

The Exception- The remote server returned an error: (403) Forbidden.

We have also confirmed the user is site collection admin under which this above code is running.

After analyzing the response in the debug mode we get the following details

{SPRequestGuid: bac0759b-03c9-4b3f-85e1-93ee7a6505f4

X-SharePointHealthScore: 0

X-Forms_Based_Auth_Required: https://sitename_login/SPORedirectMultiAuthLogin.aspx?ReturnUrl=/_layouts/Error.aspx&Source=%2fsites%2fAAAAA4882%2fAPEX%2f_vti_bin%2fsites.asmx

X-Forms_Based_Auth_Return_Url: https://sitename/_layouts/Error.aspx

X-MSDAVEXT_Error: 917656; Access denied. Before opening files in this location, you must first browse to the web site and select the option to login automatically.

MicrosoftSharePointTeamServices: 14.0.0.7106

X-Frame-Options: SAMEORIGIN

Content-Length: 13

Cache-Control: private, max-age=0

Date: Tue, 11 Feb 2014 04:53:25 GMT

Set-Cookie: spodsession=384799110.47873.0000; path=/

Server: Microsoft-IIS/7.5

X-AspNet-Version: 2.0.50727

X-Powered-By: ASP.NET

}

Regards,

Sidd