SP1 for the 3.5 Framework (and SP2 for the 2.0 Framework) added an “action” property to the HtmlForm class that allows developers to programmatically set the action attribute of the form element. Prior to this change, action attributes in existing form elements were ignored, but after the service pack is installed, ASP.NET will honor the action attribute and will begin posting the ASP.NET page to a different page. This causes the validation of the Viewstate to fail.

To avoid the exception, remove the action attribute from the form element on your ASP.NET page. Alternatively, you can use the following code in your global.asax to remove the form attribute for all of your ASP.NET pages at runtime.