Just a quick post in case someone else has ran into this issue. I was trying to set the RadFilter ExpressionPreviewPosition in OnLoad() of the RadFilter, and getting the exception
“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 fix to this is to set this property earlier, I believe OnInIt() would do it… that is all…. enjoy!