Hi,
1. Check your buttons ClientIdMode. Have you change their value to static or something else ? Try the default value.
2. Try in Page_Load to register your controls async:
ScriptManager sm = ScriptManager.GetCurrent(this.Page);
sm.RegisterAsyncPostBackControl(YourButton);
and then refresh your update panel programmatically: updatePanel.Refresh();
3. Put your buttons as triggers to your update panel:
<asp:updatepanel xmlns:asp="#unknown">
<triggers>
<asp:asyncpostbacktrigger controlid="YourButton" eventname="Click" />
</triggers>
</asp:updatepanel>
And i see that in your error there is something about a CheckBox. Do you have one on your Page ?