ubiquity does not display interface

[iGadget]

When running saucy final beta from a USB stick (option "Try Ubuntu before installing"), ubiquity does not show an interface when started. As a result, Ubuntu cannot be installed.
- Double-clicking the icon on the desktop seems to produce no result (nothing happens)
- Clicking on the 'Install Ubuntu 13.10' icon in the shortcut bar on the left side has the same result (none)
- Running "ubiquity gtk_ui" from a terminal fails:

Ubiquity is already running!

- After doing a "sudo killal ubiquity" and then running "ubiquity gtk_ui" again, no warnings are displayed, but the ubiquity user interface still does not show.

After killing ubiquity again (from another terminal, CTRL+C does not work), the terminal shows the following information:

Terminated
umount: /var/lib/polkit-1/localauthority/90-mandatory.d: not mounted

- Also tried running "ubiquity" (without gtk_ui) from the terminal -> same result (none)
- Also tried running "sh -c 'ubiquity gtk_ui'" from the terminal -> same result (none)

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: ubiquity 2.15.20
ProcVersionSignature: Ubuntu 3.11.0-8.15-generic 3.11.1
Uname: Linux 3.11.0-8-generic x86_64
ApportVersion: 2.12.4-0ubuntu1
Architecture: amd64
CasperVersion: 1.336ubuntu1
Date: Sun Oct 13 10:23:25 2013
InstallCmdLine: noprompt cdrom-detect/try-usb=true file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash --
LiveMediaBuild: Ubuntu 13.10 "Saucy Salamander" - Beta amd64 (20130925.1)
MarkForUpload: True
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)