Focal upgrade fails, can't mark packages for upgrade

[bgarnham]

Upgrade from 18.04.5 to 20.04.1 fails.

/var/log/dist-upgrade/main.log shows:

2020-10-06 20:35:21,194 DEBUG Marking 'xubuntu-core' for upgrade
2020-10-06 20:35:21,349 WARNING Can't mark 'xubuntu-core' for upgrade (E:Unable to correct problems, you have held broken packages.)
2020-10-06 20:35:21,349 DEBUG Marking 'xubuntu-desktop' for upgrade
2020-10-06 20:35:21,525 WARNING Can't mark 'xubuntu-desktop' for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)
2020-10-06 20:35:21,526 DEBUG Marking 'ubuntu-mate-core' for upgrade
2020-10-06 20:35:21,666 WARNING Can't mark 'ubuntu-mate-core' for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)
2020-10-06 20:35:21,667 DEBUG Marking 'ubuntu-mate-desktop' for upgrade
2020-10-06 20:35:21,809 WARNING Can't mark 'ubuntu-mate-desktop' for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)
2020-10-06 20:35:51,326 ERROR Dist-upgrade failed: 'Broken packages after upgrade: xubuntu-core, xubuntu-desktop'

Synaptic shows no broken packages.

$ sudo apt update --fix-missing
...All packages are up to date.

$ sudo apt install -f
...0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

$ dpkg --get-selections | grep hold
(returns nothing.)

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-118.119-generic 4.15.18
Uname: Linux 4.15.0-118-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: MATE
Date: Tue Oct 6 20:42:00 2020
InstallationDate: Installed on 2018-03-23 (928 days ago)
InstallationMedia: Xubuntu 17.10 "Artful Aardvark" - Release amd64 (20171017.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-07 (0 days ago)
VarLogDistupgradeTermlog:

[bgarnham (bgarnham)]

This was resolved by removing xubuntu-desktop and xubuntu-core, while using Mate, then running the upgrade again. xubuntu-desktop was then reinstalled, following the upgrade, and works normally.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Norbert Beckmann (norbertbn)]

I had just the same issue as described above, but trying to perform a system upgrade from 20.04 to 22.04, while working on the MATE desktop.
The solution was also just the same, i.e. removing xubuntu-desktop and xubuntu-core.
These packages are still missing, but all seems to work fine.