When developing custom action pages for the built in Ektron forms, remember to set the Page directives as specified in the Administrator Manual.
page 312
If your site is using URL Aliasing, the action page’s header must include the following:
EnableEventValidation=”false” EnableViewStateMac=”false”
Apparently when you set the EnableViewStateMac attribute to true, the encoded and encrypted view state is not checked to verify that it has not been tampered with on the client. Otherwise .NET will not persist the Viewstate from the original .NET form to the action page specified in the Ektron form.
Though disabling the EnableEventValidation attribute may be necessary, I don’t see any real reason since the page that is handling the postback shouldn’t have any controls on it in the first place. However, in the event it does, the page’s validation must be turned off else the postback handler will not validate the form post.