Hi,
That kind of error can only happen if something changed between the time the page originally loaded and when postback occurs. ie that error could be caused by loading the page in one browser window and then in another browser window changing the skin of the page. Now when the page posts back still using the old skin it would cause this error.
Other than that I know of no way to produce that error except by script bots that cause viewstate errors on purpose.
There is no known bug in the current version of Form Wizard Pro that I'm aware of, you have not provided any steps that produce the problem for me. It does not throw any errors on our demo site.
Best,
Joe