Ok, realized that I've draw the conclusion too soon.

> real cause: obsoleted package name in my class file.

no, it's actually irrelevant. The actual reason is,

fai failed due to aptitude refusing to install packages from "untrusted" source
-- now the 

 ERROR: 65280 65280 

is the only error that I have. For its cause, refer to

http://www.mail-archive.com/linux-fai@rrz.uni-koeln.de/msg03738.html

thanks