Sys.WebForms.PageRequestManagerTimeoutException
Raised by the PageRequestManager class to return an error when an asynchronous postback has exceeded the specified time for processing.
Remarks
When a request times out (when the client does not receive a response in a specified time), a PageRequestManagerTimeoutException object is created by using the Error.create function. To change the time interval that elapses before asynchronous postbacks time out if no response is received, set the AsyncPostBackTimeout property of the ScriptManager control.