Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server.

<script type="text/javascript" language="javascript">
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
    function EndRequestHandler(sender, args){
        if (args.get_error() != undefined){
            args.set_errorHandled(true);
        }
    }
</script>

Source: http://stackoverflow.com/questions/7881470/sys-webforms-pagerequestmanagerservererrorexception-an-unknown-error-occurred-w