Raised when the request times out. A partial-page update is initiated by a client request (an asynchronous postback) to the server. The server processes the request and returns a response to the client. If the browser does not receive a response in a specified time, the Sys.WebForms.PageRequestManagerTimeoutException is raised. To change the interval that elapses before asynchronous postbacks time out, set the AsyncPostBackTimeout property of the ScriptManager control.