Later in the middle-night, I checked the whizznotes/xhtml/*.html again, and found that the servlet was down again. Only after I restarted the Tomcat such 503 disappeared (Using reload servlet did not work).

The reason I guessed was that I was testing Web Services on /axis, which requiring reload the axis many times. Maybe the tomcat became unsteady when I was testing such web services.

OK, it seemed that I should make some time to adminstrate the server.