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.