This usually happens when that data that is being posted could be harmful. An example would be a textbox in which you enter HTML. Because HTML can contain javascript tags which could be harmful, ASP.NET will, by default, throw the exception you are seeing because it fails post validation. If you do as the error message suggests and set EnableEventValidation to false, this problem should go away. However, be aware that you are opening up the possibility of malicious data entry.
-Peter
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