I was just searching for a solution for this :)
it appears that you can't have OkControlID assign to a control if you want to that control fires an event, just removing this property I got everything working again.
my code (working):
<asp:Panel ID="pnlResetPanelsView" CssClass="modalPopup" runat="server" Style="display:none;">
<h2>
Warning</h2>
<p>
Do you really want to reset the panels to the default view?</p>
<div style="text-align: center;">
<asp:Button ID="btnResetPanelsViewOK" Width="60" runat="server" Text="Yes"
CssClass="buttonSuperOfficeLayout" OnClick="btnResetPanelsViewOK_Click" />
<asp:Button ID="btnResetPanelsViewCancel" Width="60" runat="server" Text="No" CssClass="buttonSuperOfficeLayout" />
</div>
</asp:Panel>
<ajax:ModalPopupExtender ID="mpeResetPanelsView" runat="server" TargetControlID="btnResetView"
PopupControlID="pnlResetPanelsView" BackgroundCssClass="modalBackground" DropShadow="true"
CancelControlID="btnResetPanelsViewCancel" />