debootstrap fails to install customized Ubuntu

When following the Install CD customization procedure (https://help.ubuntu.com/community/InstallCDCustomization) I fail to get an installable image.
Even with no changes in the CD tree, the created image fails to install. The same procedure worked before precise, I had success with natty and lucid.
The installation stops at installing base-files and base-passwd (see attached image).
If I go back to the installation menu and select Install the base system, then it succeds. Unfortunatelly, debootstrap.log gets deleted by this time, so I created a special debootstrap package with lots of debug messages. It turned out that both packages (base-files and base-passwd) got installed, but still, somehow the dpkg installing them failed.
It turned out, that base-passwd has a postinstall script that - during system installation - populates /etc/passwd and /etc/group copying the files from /usr/share/base-passwd

if [ ! -e /etc/passwd ] ; then
        cp /usr/share/base-passwd/passwd.master /etc/passwd
fi

if [ ! -e /etc/group ] ; then
        cp /usr/share/base-passwd/group.master /etc/group
fi

On the other hand, base-files also has a postinstall script changes the owner of /var/lib/dpkg
if [ ! -d /var/lib/dpkg ]; then
  mkdir -m 755 -p /var/lib/dpkg
  chown root:root /var/lib/dpkg
fi

In my log this shows as:
+ echo
+ chown: invalid user: `root:root'
dpkg: error processing base-files (--install):
 subprocess installed post-installation script returned error exit status 1
dpkg: base-passwd: dependency problems, but configuring anyway as you requested:
 base-passwd depends on libc6 (>= 2.8); however:
  Package libc6 is not installed.
Setting up base-passwd (3.5.24) ...
install base-files

which indicates that the /target/etc/passwd is not yet populated and chown cannot resolv root:root.

For some unknown reason, the postinstall script of base-files runs BEFORE the postinstall script of base-passwd in my installation, altough it does no such thing in the standard precise installation.
The code fragment that does this installation is:
  x_core_install base-files base-passwd
(in a debootstrap script called precise - a link to a long unchanged script called gutsy)

Modifying this script to have the two packages installed separatelly did the trick:
  x_core_install base-passwd
  x_core_install base-files

Now, I can install my customized distro.

I would like your opinion on what could have gone wrong about it:
Why the standard Ubuntu precise installs flawlessly while my reconstructed distro doesn't? How is it, that the same procedure used to create a custom distro suddenly went wrong with no apparent change in the components?
Ok, there might have been changes, but this area is very poorly documented so I had to trial and error to find out how things work (and I still might be wrong).