ubiquity crashed with DebconfError in command(): (10, "debian-installer/fallbacklocale doesn't exist")

Binary package hint: ubiquity

RELEASE: Natty Narwhal daily-live image
CD/DVD VARIANT: 64bit Xubuntu desktop cd
ISO BUILD: 2011-03-16
SYMPTOMS:
After starting the live desktop, I clicked on " Install Xubuntu 11.04". Ubiquity crashes and the installer does not start.

STEPS TO REPRODUCE:
1. Download the Xubuntu Natty Narwhal 64bit image dated 2011-03-16
2. Choose a language
3. Choose " Try without installing"
4. At the live desktop, double-click the installer icon

ProblemType: Crash
DistroRelease: Ubuntu 11.04
Package: ubiquity 2.5.26
ProcVersionSignature: Ubuntu 2.6.38-6.34-generic 2.6.38-rc7
Uname: Linux 2.6.38-6-generic x86_64
Architecture: amd64
Date: Wed Mar 16 13:36:47 2011
ExecutablePath: /usr/lib/ubiquity/bin/ubiquity
InterpreterPath: /usr/bin/python2.7
LiveMediaBuild: Xubuntu 11.04 "Natty Narwhal" - Alpha amd64 (20110316)
ProcCmdline: /usr/bin/python /usr/lib/ubiquity/bin/ubiquity gtk_ui
ProcEnviron: Error: [Errno 13] Permission denied: '/proc/3049/environ'
PythonArgs: ['/usr/lib/ubiquity/bin/ubiquity', 'gtk_ui']
SourcePackage: ubiquity
Title: ubiquity crashed with DebconfError in command(): (10, "debian-installer/fallbacklocale doesn't exist")
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: