How to resolve (OAuthException - #190) Error validating access token: This may be because the user logged out or may be due to a system error

I am trying to post on my Facebook fan page using Facebooh Graph API. To do this I am using extended Page access token. I was able to post my contents on Facebook, but the moment I logged out from administrator account and tried posting the contents on my Fan Page I am getting this error. Please guide me how can I resolve this issue?

To get Extended Page Access token I have followed these processes-

1. Created an FB App to get AppID and AppSecret.


2. Headed over to Facebook Graph API Explorer


3. On the top right in "Application" I have selected my app which I have created.


4. Clicked on Get Access Token Button


5. Added "manage_pages" permission.


6. Converted short-lived access token into a long lived one by making this Graph API http://ift.tt/1bsunJu LIVED TOKEN


7. Grabbed new long lived access token returned back. Now I am using this long lived access token in my code. Please guide me how can I resolve this issue.

CS Code-

    protected void Page_Load(object sender, EventArgs e)
    {
        CheckAuthorization();
    }

    private void CheckAuthorization()
    {
        string app_id = "XXXXXX";
        string app_secret = "XXXXX";
        //string my_url = "http://localhost:6943/ashish/Default2.aspx";
        string scope = "publish_stream, publish_actions, manage_pages, email,user_location,user_birthday";
        if (Request["code"] == null)
        {
            Response.Redirect(string.Format("http://ift.tt/IElFZM}", app_id, Request.Url.AbsoluteUri, scope));
        }
        else
        {
            Dictionary<string, string> tokens = new Dictionary<string, string>();
            string url = string.Format("http://ift.tt/IElGg0}", app_id, Request.Url.AbsoluteUri, scope, Request["code"].ToString(), app_secret);
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            StreamReader reader = new StreamReader(response.GetResponseStream());

            string vals = reader.ReadToEnd();

            foreach (string token in vals.Split('&'))
            {
                tokens.Add(token.Substring(0, token.IndexOf("=")),
                    token.Substring(token.IndexOf("=") + 1, token.Length - token.IndexOf("=") - 1));
            }
            string access_token = "Long Lived Access Token";
var client = new FacebookClient(access_token);
            dynamic parameters = new ExpandoObject();
            parameters.message = "xxxxxxxxxxxxxxxxxxxxxxxx";
            parameters.link = "XXXXXXXXXXXX";
            parameters.picture = "XXXXXXXXXXXXXXXXX";
            parameters.name = "XXXXXXXXXXXXXXX";
            parameters.caption = "XXXXXXXXXXX";
client.Post("/FB Fan page ID/feed", parameters);
}
    }