Strangely enough it is not related to RadAjaxPanel. It is a button that exports from RadGrid to Excel. There is one line of code behind it:
Grid.MasterTableView.ExportToExcel();
This works fine under ASP.Net development server, but as soon as control is deployed to SharePoint, the export works only once, second time page does not post back. Here is simplified page markup:
<telerik:RadTabStrip ID="TopTabStrip" runat="server" MultiPageID="MultiPage" Skin="Windows7" SelectedIndex="0"
OnTabClick="OnTabClicked">
<Tabs>
<telerik:RadTab runat="server" Text="Shipments" />
<telerik:RadTab runat="server" Text="Documents" />
</Tabs>
</telerik:RadTabStrip>
<telerik:RadMultiPage ID="MultiPage" runat="server" SelectedIndex="0">
<telerik:RadPageView ID="ItemsView" runat="server">
<div style="float:right">
<asp:ImageButton ID="ExportToExcel" runat="server" ImageUrl="~/_layouts/images/excel_export.png"
AlternateText="Export"
OnClick="OnExportToExcel" />
</div>
<telerik:RadGrid id="ItemsGrid" runat="server" AutoGenerateColumns="false" Skin="Windows7"
OnNeedDataSource="OnItemsGridNeedDataSource" AllowSorting="true" AllowPaging="true" PageSize="20">
<MasterTableView ItemStyle-Wrap="false" HeaderStyle-Wrap="false">
<Columns>
</Columns>
</MasterTableView>
<ExportSettings OpenInNewWindow="true" IgnorePaging="true" ExportOnlyData="true" FileName="ShipmentLineItems" />
</telerik:RadGrid>
</telerik:RadPageView>
-Stan