the failling version is ubiquity 2.18.8.2.

I am using an updated live-cd from an official kubuntu-14.04.1-desktop-amd64

Using the official live-cd as is, doesn't reproduce the error. 

But updating ubiquity (intialy 2.18.8) in a live session, as below, reproduce the same problem:

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kubuntu@kubuntu:~$ ubiquity --version
2.18.8
kubuntu@kubuntu:~$ sudo apt-get update
(...)
kubuntu@kubuntu:~$ sudo apt-get install ubiquity
Lecture des listes de paquets... Fait
Construction de l'arbre des dépendances       
Lecture des informations d'état... Fait
Les paquets supplémentaires suivants seront installés : 
  dconf-cli gir1.2-appindicator3-0.1 gir1.2-atk-1.0 gir1.2-freedesktop
  gir1.2-gdkpixbuf-2.0 gir1.2-glib-2.0 gir1.2-gtk-3.0 gir1.2-ibus-1.0
  gir1.2-javascriptcoregtk-3.0 gir1.2-json-1.0 gir1.2-pango-1.0
  gir1.2-timezonemap-1.0 gir1.2-vte-2.90 gir1.2-webkit-3.0 gir1.2-xkl-1.0
  gnome-desktop3-data gnome-icon-theme gnome-session gnome-session-bin
  gnome-session-common gnome-settings-daemon gnome-settings-daemon-schemas
  humanity-icon-theme hwdata ibus indicator-application libappindicator3-1
  libcanberra-gtk-module libcanberra-gtk0 libcanberra-gtk3-0
  libcanberra-gtk3-module libdbusmenu-gtk3-4 libgeoclue0 libgirepository-1.0-1
  libgnome-desktop-3-7 libido3-0.1-0 libindicator3-7
  libjavascriptcoregtk-3.0-0 libjson-glib-1.0-0 libjson-glib-1.0-common
  libmetacity-private0a libnotify4 libpangoxft-1.0-0 librsvg2-2
  librsvg2-common libsystemd-journal0 libtimezonemap1 libvte-2.90-9
  libvte-2.90-common libwacom-common libwacom2 libwebkitgtk-3.0-0
  libwebkitgtk-3.0-common metacity metacity-common nautilus-data python-cairo
  python-gtk2 python-notify python3-cairo python3-gi python3-gi-cairo
  session-migration ubiquity-frontend-gtk ubiquity-frontend-kde
  ubiquity-ubuntu-artwork zenity zenity-common
Paquets suggérés :
  gnome-user-guide desktop-base gnome-screensaver ibus-clutter ibus-doc
  geoclue librsvg2-bin gnome-control-center gnome-themes-standard nautilus
  python-gtk2-doc
Les NOUVEAUX paquets suivants seront installés :
  dconf-cli gir1.2-appindicator3-0.1 gir1.2-atk-1.0 gir1.2-freedesktop
  gir1.2-gdkpixbuf-2.0 gir1.2-gtk-3.0 gir1.2-ibus-1.0
  gir1.2-javascriptcoregtk-3.0 gir1.2-json-1.0 gir1.2-pango-1.0
  gir1.2-timezonemap-1.0 gir1.2-vte-2.90 gir1.2-webkit-3.0 gir1.2-xkl-1.0
  gnome-desktop3-data gnome-icon-theme gnome-session gnome-session-bin
  gnome-session-common gnome-settings-daemon gnome-settings-daemon-schemas
  humanity-icon-theme hwdata ibus indicator-application libappindicator3-1
  libcanberra-gtk-module libcanberra-gtk0 libcanberra-gtk3-0
  libcanberra-gtk3-module libdbusmenu-gtk3-4 libgeoclue0 libgnome-desktop-3-7
  libido3-0.1-0 libindicator3-7 libjavascriptcoregtk-3.0-0 libjson-glib-1.0-0
  libjson-glib-1.0-common libmetacity-private0a libnotify4 libpangoxft-1.0-0
  librsvg2-2 librsvg2-common libsystemd-journal0 libtimezonemap1 libvte-2.90-9
  libvte-2.90-common libwacom-common libwacom2 libwebkitgtk-3.0-0
  libwebkitgtk-3.0-common metacity metacity-common nautilus-data python-cairo
  python-gtk2 python-notify python3-cairo python3-gi-cairo session-migration
  ubiquity-frontend-gtk zenity zenity-common
Les paquets suivants seront mis à jour :
  gir1.2-glib-2.0 libgirepository-1.0-1 python3-gi ubiquity
  ubiquity-frontend-kde ubiquity-ubuntu-artwork
6 mis à jour, 63 nouvellement installés, 0 à enlever et 348 non mis à jour.
Il est nécessaire de prendre 22,4 Mo dans les archives.
Après cette opération, 84,9 Mo d'espace disque supplémentaires seront utilisés.
Souhaitez-vous continuer ? [O/n] O

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You can found attached the output of "ubiquity --debug"