Check you GridView can be DoubleBind being of this, generated Row ID become change of each GridView row.
I think being of this generated Row IDs in the Postback seems to differ from the row id generated in the initial post.
Invalid postback or callback argument in GridView ImageButton
If you are using Imagebutton inside GrideView and on click you are getting Invalid postback or callback then below reason can be responsible for this...
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