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

 

http://weblogs.asp.net/alaaalnajjar/archive/2010/05/06/solved-failed-to-load-viewstate-the-control-tree-into-which-viewstate-is-being-loaded-etc.aspx

http://forums.asp.net/t/1295517.aspx/1