formsauthentication.decrypt returns null in server with 4.5 version installed

Hello,

I have problem with Decrpt method of FormsAuthentication.
I have 2 server 2003  for my website: 
Server 1 with 4.0 Framework installed
Server 2 with 4.5 Framework installed

The probleme is server 2 return always null for this :
FormsAuthentication.Decrypt(authCookie.Value) In my web.config i have the same MachineKey and i have added the compatibilityMode 
<system.web>      <machineKey validationKey="...." decryptionKey="...." validation="SHA1" compatibilityMode="Framework20SP1" /> Can any one help me ?

Thx.