apt-get error=100

When running uck-gui a copy of the scripts found in /usr/lib/uck/customization-profiles/localized_cd gets copied to ~/tmp/customization scripts unconditionally. If you need to modify the behaviour of the "customize" script run during customization of the rootfs, the easiest approach is to modify the "customize" procedure in the directory specified above so your version gets used in place of the original version from the distribution (your change will be overwritten by an upgrade/reinstall of the package, though).

As an alternative (if you already have run customization at least once, even if it failed): Move ~/tmp/customization-scripts to ~/tmp/customization-scripts.local, modify the "customize" procedure in ~/tmp/customization-scripts.local/customize and the run "uck-remaster location-of-your-iso-file.iso ~/tmp/customization-scripts.local". This will use your locally modified copy of the customization scripts instead of the default ones.

In both cases the function you want to modify is:
function prepare_install()
{
        #try 2 times to avoid slow proxies failures
        apt-get update || apt-get update || failure "apt-get update failed, error=$?"
}
and the modifcation you want to make is:
function prepare_install()
{
       run_console
        #try 2 times to avoid slow proxies failures
        apt-get update || apt-get update || failure "apt-get update failed, error=$?"
}
which will give you a console right before apt-get update is called, allowing you to remove the
offending repository at that point.

Hope this helps.