lh build: How-to debug Sub-process /usr/bin/dpkg returned an error code (1)

[Philippe Lelédy <debian@leledy.fr>]

live_helper: 2.0~a16.1

After:

lh config --mode ubuntu -d lucid [...]
lh build

and some littles fixes due to little divergences btw debian & ubuntu 
packages, things seam to go fine with more than 1000 packages installed, 
but after successful

Generating /boot/initrd.img-2.6.32-22-generic

and

Rebuilding /usr/share/applications/desktop-POSIX.cache
[...]
Processings triggers for python-[...]

comes this error:

Errors were encountered while processing:
desktop-base
gnome-desktop-environment
E: Sub-process /usr/bin/dpkg returned an error code (1)

and build stop with RC=123


I don't know where to go for debugging.

Is there a way to manually chroot for manually finish the job in order 
to find out how to fix the issue before trying yet another (long) run ?

Thks.

Ph. Lelédy