A few days ago, I wanted to add an Export to Excel button in a page in my web application, that I used Ajax on it, But unfortunately it did not work properly and generated an error(the same as my post header). After searching in the net I found out I have to add a line in page. That's it, it Worked excellent :). If you encountered this error, you must add:
EnableEventValidation="false"
and also:
<Triggers>
<asp:PostBackTrigger ControlID="ControlName" />
</Triggers>
Good luck :)
With a wabi-sabi mindset, let's code!