Originally when this site was setup with Drupal, the system configuration met the requirements of clean url by Drupal. Later, due to some reason unknown, the system configuration does not meet the requirement any more. The consequence was that the whole sitewas almost down: every address resulted in Error 404. Login as administrator became extremely difficult. The rewrite of URL was a nightmare.
Now, on 10. Dec 2008, clean URL was disabled. The system was basically recovered.