There are 2 groups of computers (152 of them) on my network suddenly start to have update failure:
updates were not applied because of a detection script download/verification error
This happen around April 26, 2012, as the last definition was 6693.
There are still a few of them able to update successfully.
All the computers are Windows XP Professional.
Viruscan version is 8.7
Firewall administrator had verified that no rule blocking the communication between the clients and server.