Unable to network upgrade from 9.10 to 10.04

[Nikki!]

Binary package hint: update-manager

I started the upgrade and got this error during the Setting new software channels phase:

=========================
Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade:
The package 'ubuntu-desktop' is marked for removal but it is in the removal blacklist.

 This can be caused by:
 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu
 * Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug against the 'update-manager' package and include the files in /var/log/dist-upgrade/ in the bug report.
=========================

* I'm not upgrading to a pre-release version
* I'm not running a pre-release version

There's also this message:

    Dist-upgrade failed: 'The package 'ubuntu-desktop' is marked for removal but it is in the removal blacklist.'

My system was originally Ubuntu 9.10, over which I installed xubuntu-desktop.

ProblemType: Bug
Architecture: i386
Date: Sun May 2 09:12:53 2010
DistroRelease: Ubuntu 9.10
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release i386 (20091028.2)
NonfreeKernelModules: nvidia
Package: update-manager 1:0.126.9
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, user)
 LANG=en_PH.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-21.59-generic
SourcePackage: update-manager
Uname: Linux 2.6.31-21-generic i686
XsessionErrors:
 (xfce4-cpugraph-plugin:2423): Gtk-CRITICAL **: gtk_container_set_border_width: assertion `GTK_IS_CONTAINER (container)' failed
 (nautilus:2424): Eel-CRITICAL **: eel_preferences_get_boolean: assertion `preferences_is_initialized ()' failed
 (polkit-gnome-authentication-agent-1:2437): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed
 (xfce4-terminal:5194): Terminal-WARNING **: Unable to load terminal preferences.