Why I got this exception :Sys.WebForms.PageRequestManagerServerErrorException: Invalid postback or callback argument

Why I got this error

Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: 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.

this error appears when the event for Save command is started

Protected Sub DataList1_ItemCommand(source As Object, e As System.Web.UI.WebControls.DataListCommandEventArgs) Handles dlMainTraing.ItemCommand
If e.CommandName = "Save" Then
Dim lblMainId = CType(dlMainTraing.Items(e.Item.ItemIndex).FindControl("Train_M_IndxLabel"), Label)
Dim txtSubTrainingName2 = CType(dlMainTraing.Items(e.Item.ItemIndex).FindControl("txtSubTrainingName"), TextBox)
Dim a = addSubTraining(CInt(lblMainId.Text), txtSubTrainingName2.Text)
' Dim gvSubTraning As GridView = CType(e.Item.FindControl("gvSubTraning"), GridView)
'gvSubTraning.DataBind()
End If

End Sub

At the top of your .aspx page, set EnableEventValidation to false.

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