I too have just run into the same error message. The DNN error log reads:
Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.
The form was redirecting without any issues, and I can't pinpoint what I modified before the error message started, but here are the changes I've made to the form:
1. deleted a radio button question
2. modified the "Submit" link to read "Continue..." I have since switched it back to read "Submit"
3. Delete two "Question Events"
4. Modified questions from using a single row per question, to three questions per row.
I'm running version 2.3.7 on DNN 4.06. The redirect for the form is another page on my site. I tried doing a redirect to an external web site (www.google.com) just to see if that would work , but it did not. I look forward to hearing the solution to this "unplanned" functionality. - Regards, Zamo