Restore the final Debian repository URL
In the file
config-base/bootstrap, the parameter LB_MIRROR_BINARY on line 54 controls the repository URL that is ending in the file
/etc/apt/sources.list of the CD. It should be
%MIRROR% as it was in the Lenny based version since a build script is then replacing it with a repository that is nearer to the user's location (assumption made on language, of course).