Why do I have an error message when deleting record from Datagrid ?

[Robert Chu]

Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> 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.

[drallab]

Try adding " EnableEventValidation="false"" to your page directive "<%@ Page %>" in the source.