Please add this code in your web.config page :
<system.web>
<machinekey validation="SHA1" validationkey="A1B2C3D4E5F6F6E5D4C3B2A1A1B2C3D4E5F6F6E5D4C3B2A1A1B2C3D4E5F6F6E5D4C3B2A1A1B2C3D4E5F6F6E5D4C3B2A1A1B2C3D4E5F6F6E5D4C3B2A1B2C3D4E5">
decryption="Auto" decryptionKey="A1B2C3D4E5F6F6E5D4C3B2A1A1B2C3D4E5F6F6E5D4C3B2A1" />
</machinekey></system.web>