The default value of AJAX postback is 90 seconds, so if you have a long asynchronous postback, you will get an error as this:

Sys.WebForms.PageRequestManagerTimeoutException: The server request timed out.

The fix is to change the default timeout value to a larger value in your ScriptManger’s AsyncPostBackTimeout property (10 minutes as below):

<asp:ScriptManager ID="ScriptManager1" runat="server" AsyncPostBackTimeout="600">
        </asp:ScriptManager>