This bug was reported to Ubuntu Launchpad as: 
https://bugs.launchpad.net/bugs/532537

From the launchpad reporter: 

It's just a feeling, but I think xfce4-clipman needs more time to exit when logging off.

Description: Ubuntu lucid (development branch) Release: 10.04
(Xubuntu really)

xfce4-clipman:
  Installed: 2:1.1.3-1
  Candidate: 2:1.1.3-1
  Version table:
 *** 2:1.1.3-1 0
        500 http://nl.archive.ubuntu.com lucid/universe Packages
        100 /var/lib/dpkg/status

ProblemType: Crash
Architecture: i386
CrashCounter: 1
Date: Fri Mar 5 11:10:07 2010
DistroRelease: Ubuntu 10.04
ExecutablePath: /usr/bin/xfce4-clipman
InstallationMedia: Xubuntu 10.04 "Lucid Lynx" - Alpha i386 (20100224.1)
Package: xfce4-clipman 2:1.1.3-1
ProcCmdline: xfce4-clipman
ProcEnviron:
 LANG=en_US.utf8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.32-15.22-generic-pae
SegvAnalysis:
 Segfault happened at: 0x8050336: mov 0x4(%esi),%ecx
 PC (0x08050336) ok
 source "0x4(%esi)" (0x00000004) not located in a known VMA region (needed readable region)!
 destination "%ecx" ok
SegvReason: reading NULL VMA
Signal: 11
SourcePackage: xfce4-clipman-plugin
StacktraceTop:
 ?? ()
 g_cclosure_marshal_VOID__BOXED ()
 g_closure_invoke () from /usr/lib/libgobject-2.0.so.0
 ?? () from /usr/lib/libgobject-2.0.so.0
 g_signal_emit_valist () from /usr/lib/libgobject-2.0.so.0
Title: xfce4-clipman crashed with SIGSEGV in g_cclosure_marshal_VOID__BOXED()
Uname: Linux 2.6.32-15-generic-pae i686
UserGroups: adm admin cdrom dialout lpadmin plugdev sambashare
XsessionErrors:
 (polkit-gnome-authentication-agent-1:7013): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed
 (tomboy:6997): Gtk-WARNING **: Inserting action group 'Notebooks' into UI manager which already has a group with this name

Stacktrace:
http://launchpadlibrarian.net/40237666/Stacktrace.txt

ThreadStacktrace:
http://launchpadlibrarian.net/40237667/ThreadStacktrace.txt