I have a ASP.Net web application that utilizes the Report Server control. Certain reports were timing out when run from the application, yet did not timeout when run from Report Manager. The error appeared as a JavaScript parameter count message. The problem was solved by adding the AsyncPostBackTimeout attribute to the AJAX control on the page. The default timeout is 90 seconds. The modified AJAX control tag now looks like this:
<asp:ScriptManager ID="ScriptManager1" AsyncPostBackTimeout="180" runat="server" />
This sets the timeout to 3 minutes.