Trying to prevent session tampering works on local environment but not on prod server - asp.net

Question

So i want to prevent session tampering in my site and i implemented this in global.asax. What im doing is im generating a hash key using the GenerateHashKey function. which basically uses the browser version,userhost address etc to create a hash key. This hash key im attaching to ASP.NET_SessionId cookie. Now this works perfectly in local environment. but as soon as i host it to prod server, the "Invalid" exception is thrown the first time and then it works fine. why is this happening
I used this article
http://www.codeproject.com/Articles/859579/Hack-proof-your-asp-net-applications-from-Session
protected void Application_BeginRequest(object sender, EventArgs e)
{
try
{
if (Request.Cookies["ASP.NET_SessionId"] != null && Request.Cookies["ASP.NET_SessionId"].Value != null)
{
string newSessionID = Request.Cookies["ASP.NET_SessionId"].Value;
//Check the valid length of your Generated Session ID
if (newSessionID.Length <= 24)
{
//Log the attack details here
Response.Cookies["ASP.NET_SessionId"].Expires = DateTime.Now.AddDays(-30);
Response.Cookies["ASP.NET_SessionId"].Value = null;
throw new HttpException("Empty");
}
//Genrate Hash key for this User,Browser and machine and match with the Entered NewSessionID
if (GenerateHashKey() != newSessionID.Substring(24))
{
//Log the attack details here
Response.Cookies["TriedTohack"].Value = "True";
Response.Cookies["ASP.NET_SessionId"].Expires = DateTime.Now.AddDays(-30);
Response.Cookies["ASP.NET_SessionId"].Value = null;
throw new HttpException("Invalid:"+newSessionID);
}
//Use the default one so application will work as usual//ASP.NET_SessionId
Request.Cookies["ASP.NET_SessionId"].Value = Request.Cookies["ASP.NET_SessionId"].Value.Substring(0, 24);
}
}
catch(Exception Ex)
{
if (Ex.Message == "Invalid")
{
Response.Redirect(string.Format("~/PraiseError.aspx?Message={0}", Uri.EscapeDataString(Ex.Message)));
}
else
{
Response.Redirect("~/Home.aspx");
}
}
}
protected void Application_EndRequest(object sender, EventArgs e)
{
string gn = GenerateHashKey();
try
{
//Pass the custom Session ID to the browser.
if (Response.Cookies["ASP.NET_SessionId"] != null)
{
Response.Cookies["ASP.NET_SessionId"].Value = Request.Cookies["ASP.NET_SessionId"].Value.Replace(gn, "") + gn;
}
else
{
Response.Cookies["ASP.NET_SessionId"].Value = Request.Cookies["ASP.NET_SessionId"].Value + gn;
}
}
catch
{
Response.Cookies["ASP.NET_SessionId"].Value = Request.Cookies["ASP.NET_SessionId"].Value + gn;
}
}
private string GenerateHashKey()
{
StringBuilder myStr = new StringBuilder();
myStr.Append(Request.Browser.Browser);
myStr.Append(Request.Browser.Platform);
myStr.Append(Request.Browser.MajorVersion);
myStr.Append(Request.Browser.MinorVersion);
myStr.Append(Request.UserHostAddress);
//myStr.Append(Request.LogonUserIdentity.User.Value);
SHA1 sha = new SHA1CryptoServiceProvider();
byte[] hashdata = sha.ComputeHash(Encoding.UTF8.GetBytes(myStr.ToString()));
return Convert.ToBase64String(hashdata);
}