The most common cause of this problem is the time of asyncronous response. It occur when the response takes more than 90 seconds. First of all be sure that there is no client script in the update panel. Use
to register such scripts during loading page.
If the reason is long calculations on the srever you can change the asyncronous timeout in the ScriptManager tag.
<asp:ScriptManager ID="ScriptManager1" AsyncPostBackTimeOut="3600" runat="server" />
This makes the response to be valid an hour.