I am sorry. I know this question has been asked many times and I have read so many articles but still could not fix the issue. Here I put everything together.
http://forums.asp.net/t/1212323.aspx
http://odetocode.com/blogs/scott/archive/2006/03/21/asp-net-event-validation-and-invalid-callback-or-postback-argument-again.aspx
http://johanleino.wordpress.com/2009/11/17/cascadingdropdown-causes-invalid-postback-or-callback-argument-error/
http://stackoverflow.com/questions/228969/invalid-postback-or ...
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