FormsAuthentica tion.Decryption problem

[Rodrigo m. Ferreira]

Can you help me to solve the following problem?
on my loggin page I have the code:

protected void LoginButton_Cli ck(object sender, EventArgs e)
{
if(Membership.V alidateUser(TXT Usuario.Text, TXTSenha.Text))

{

if (CBLembra.Check ed)
{
FormsAuthentica tionTicket ticket = new
FormsAuthentica tionTicket(1,TX TUsuario.Text,
DateTime.Now,Da teTime.Now.AddD ays(7),true,"Co okieUsuario") ;
HttpCookie OuttaBiscoito = new
HttpCookie("Out taBiscoito");
OuttaBiscoito.V alue =
System.Web.Secu rity.FormsAuthe ntication.Encry pt(ticket);
OuttaBiscoito.E xpires = DateTime.Now.Ad dDays(7);
Response.Cookie s.Add(OuttaBisc oito);

}
else
{
Response.Cookie s.Remove("Outta Biscoito");
}

FormsAuthentica tion.RedirectFr omLoginPage(TXT Usuario.Text,
false );

}

on my Global.asax:

protected void Session_Start(o bject sender, EventArgs e)
{

HttpCookie OuttaBiscoito = Request.Cookies["OuttaBisco ito"];
if (OuttaBiscoito != null)
{
FormsAuthentica tionTicket ticket =
FormsAuthentica tion.Decrypt(Ou ttaBiscoito.Val ue);
FormsAuthentica tion.SetAuthCoo kie(ticket.Name ,
false, ticket.CookiePa th);
FormsAuthentica tionTicket ticket2 = new
FormsAuthentica tionTicket(1, ticket.Name, DateTime.Now,
DateTime.Now.Ad dDays(7), true, "CookieUsuario" );
OuttaBiscoito.V alue =
FormsAuthentica tion.Encrypt(ti cket2);
OuttaBiscoito.E xpires = DateTime.Now.Ad dDays(7);
Response.Cookie s.Add(OuttaBisc oito);
}
}

If i loggin and exit browser when i acess again it goes OK, but only in a
few minutes. If i exit the browser and wait about half an hour to acess, the
following error is showed:

Server Error in '/outtacom' Application.
--------------------------------------------------------------------------------

Padding is invalid and cannot be removed.
Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information
about the error and where it originated in the code.

Exception Details: System.Security .Cryptography.C ryptographicExc eption:
Padding is invalid and cannot be removed.

Source Error:

An unhandled exception was generated during the execution of the current web
request. Information regarding the origin and location of the exception can
be identified using the exception stack trace below.

Stack Trace:
[CryptographicEx ception: Padding is invalid and cannot be removed.]
System.Security .Cryptography.R ijndaelManagedT ransform.Decryp tData(Byte[]
inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer,
Int32 outputOffset, PaddingMode paddingMode, Boolean fLast) +1545747
System.Security .Cryptography.R ijndaelManagedT ransform.Transf ormFinalBlock(B yte[]
inputBuffer, Int32 inputOffset, Int32 inputCount) +257
System.Security .Cryptography.C ryptoStream.Flu shFinalBlock() +30
System.Web.Conf iguration.Machi neKeySection.En cryptOrDecryptD ata(Boolean
fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Boolean
useValidationSy mAlgo) +164
System.Web.Secu rity.FormsAuthe ntication.Decry pt(String encryptedTicket )
+242
AdminSite.Globa l.Session_Start (Object sender, EventArgs e) in
Global.asax.cs: 29
System.Web.Sess ionState.Sessio nStateModule.Ra iseOnStart(Even tArgs e)
+2221855
System.Web.Sess ionState.Sessio nStateModule.Co mpleteAcquireSt ate() +159
System.Web.Sess ionState.Sessio nStateModule.Be ginAcquireState (Object
source, EventArgs e, AsyncCallback cb, Object extraData) +518
System.Web.Asyn cEventExecution Step.System.Web .HttpApplicatio n.IExecutionSte p.Execute()
+90
System.Web.Http Application.Exe cuteStep(IExecu tionStep step, Boolean&
completedSynchr onously) +161


--------------------------------------------------------------------------------
Version Information: Microsoft .NET Framework Version:2.0.507 27.1434;
ASP.NET Version:2.0.507 27.1434