Hi Richard,
I have fixed the problem by reinstalling the USB Blaster driver of rev.20.4 (rev.19.1 was failed with " file corrupted" info). Partially reason, I guess, might be that I did not have administrator level authority for the machine. The system just blocks it. But, I still don't know why it was working at the beginning when I installed the Quartus two years ago and later it became failure. Perhaps it was intervened by some security software installation since then. What I did today was just wiped out the driver totally and reinstall it with the admin authority. It works now. Weird, isn't it?
Anyway, very appreciate your helps!
Yaoting Yang
AEM America, Inc.