X crash at login: polkit-gnome-authentication-agent-1 crashed with SIGSEGV in g_datalist_id_set_data_full()

In Ubuntu 11.04, when I login using "Ubuntu" or "Ubuntu classic" mode, I have an immediate crash. Here I have filled this report using apport-bug informaiton.

Note that when I login using "Ubuntu classic (without effect)", I don't have the crash and the desktop seems to work fine.

This bug report is the same as this one I already filled https://bugs.launchpad.net/bugs/793114
But here you have the apport crash report.
I hope it will help to find out the problem.

The crash is in fact in X server, as you can see in the attached Xorg.0.log.old

ProblemType: Crash
DistroRelease: Ubuntu 11.04
Package: policykit-1-gnome 0.99-1ubuntu4
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic x86_64
Architecture: amd64
CrashCounter: 1
Date: Sun Jun 12 16:06:41 2011
ExecutablePath: /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
ProcCmdline: /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1
ProcCwd: /home/loic
ProcEnviron:
 LANGUAGE=fr_FR:en
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SegvAnalysis:
 Segfault happened at: 0x7f3d982229d8 <g_datalist_id_set_data_full+744>: cmp 0x8(%rax),%r12d
 PC (0x7f3d982229d8) ok
 source "0x8(%rax)" (0x7375424437) not located in a known VMA region (needed readable region)!
 destination "%r12d" ok
SegvReason: reading unknown VMA
Signal: 11
SourcePackage: policykit-1-gnome
StacktraceTop:
 g_datalist_id_set_data_full () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
 g_object_unref () from /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
 ?? ()
 __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
 ?? ()
Title: polkit-gnome-authentication-agent-1 crashed with SIGSEGV in g_datalist_id_set_data_full()
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm admin cdrom dialout lpadmin plugdev sambashare