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.
Lifecycle of a Web Form
ViewState and its role in ASP.NET page processing
Stage 0 - Instantiation
Stage 1 - Initialization
Stage 2 - Load View State
Stage 3 - Load Postback Data
Stage 4 - Load
Stage 5 - Raise Postback Event
Stage 6 - Save View State
Stage 7 - Render