Problem with Validation of viewstate MAC failed

[Spondishy]

Hi,

I'm running an application that is getting the Validation of viewstate
MAC failed error. This occurs if the application has been left for a
long period of time and a user performs a post-back. This is an
anonymous app (no login required).

I've added the following line to the web.config, but the error still
occurs.

<pages enableEventValidation="false" viewStateEncryptionMode ="Never"
/>

Am I missing something to stop this error occuring. Also if the app
moves to a cluster, do I need to do anything else, I assume I don't
need to implement machine keys as the viewstate is supposedly
unencrypted.

Thanks in advance.