I started getting the error below when using markup inside of an UpdatePanel. No error is shown at the browser but we have an email sent when page errors occur. I am not sure where to look to resolve this error. Help.
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.