AJAX ERROR : Sys.WebForms.PageRequestManagerTimeoutException - The server request timed out

Well if you have this error then its due to long running process as the default time out set is 90 seconds.
If you want to increase the time out value , then make change to the Script Manager attribute .
For e.g to increase it to 5 min :




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Configuring AJAX in ASP.NET

Here is the list of web.config entries requires for AJAX to work in ASP.NET App.

http://www.asp.net/AJAX/documentation/live/ConfiguringASPNETAJAX.aspx