Invalid postback or callback argument.  Event validation is enabled using <pages enableeventvalidation="true"></pages> 
in configuration or <%@ Page EnableEventValidation="true" %> in a page.  For security purposes, this feature
 verifies that arguments to postback or callback events originate from the server control that originally rendered 
them.  If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order 
to register the postback or callback data for validation.

Description: An unhandled exception occurred during the execution of the current web request. Please review the 
stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: Invalid postback or callback argument.  Event validation is 
enabled using <pages enableeventvalidation="true"></pages> in configuration or <%@ Page EnableEventValidation="true" %> 
in a page.  For security purposes, this feature verifies that arguments to postback or callback events originate from the 
server control that originally rendered them.  If the data is valid and expected, use the 
ClientScriptManager.RegisterForEventValidation method in or



Its easy to fix try this 
This could easily be fixed

just add enableeventvalidation="false" in the top most line <%@ Page