Got this error message when trying to do a reprepro update:
aptmethod error receiving 'http://security.debian.org/debian-security/dists/lenny/updates/main/debian-installer/binary-i386/Packages.gz': '404 Not Found'
This was caused by a missing UDebComponents: none in the security update source definition, in conf/updates. Quite obvious really, but got me scratching my head for a while.