Comment 5 for bug 820514

This appears to be because ubiquity-frontend-gtk and oem-config-debconf are on the CD. Ubiquity running in oem-config mode will see that the GTK frontend is present and try to use that, but because you don't have oem-config-gtk installed, it will be missing files necessary to complete the install (like oem-config-remove-gtk).

The solution is to seed oem-config-gtk on your images, or if you really want to use the debconf frontend, remove ubiquity-frontend-gtk from the image seed.