About Form Authentication SSO across IIS application pools

Question

I have a question about Form Authentication across IIS application pools.

I have 2 web applications, they are running under different application pools, and application pools are configured with same identify, and launch another web application with Form Authentication SSO.

The MachineKey configuration in web.config is as follows,

<machineKey validationKey="AutoGenerate" decryptionKey="AutoGenerate" validation="3DES" decryption="3DES"/>

The Form Authentication SSO is failed when my applications are running on pure x64 mode on Windows 2008 x64 OS. And the same program can work on win32 mode on Windows 2008 x64 OS.

As I know, if the application pools configured with same identity, we can use AutoGenerate in machineKey setting. I did more investigation, when I configured fixed machine key in web.config, the Form Authentication SSO go to work in pure x64 mode.

<machineKey validationKey="7F23AF6F8FBF16BD74C17D760590FA990697394942E145F033B6A3474E492EB76EAC4D62403CC50ADF481C8AB031D3A60AD53C504D63412976D50B2814DDC31A" decryptionKey="13114A27C3F6F5A3DC4A1E37FB21B8C0E8EB0AE94FCACA42" validation="3DES"/>

Could you help to clarify why this issue only occurred in pure x64 mode?

How can I resolve this issue?

Thanks in advance!

Best Regards