Ubuntu 20.04 update from 18.04 freezed then partial_update fails

[Remi Chartier]

- Running on Lubuntu 18.04.
- Got software upgrade popup to upgrade to 20.04.
- Answered yes, started the update progress.
- After about 10 minutes, got a popup window, asking to select one choice only like "lightDM" but impossible to answer as this pop-up window was frozen, everything else was frozen.
- Proceeded to turnoff laptop (long push 10s on power button).
- Turnon Laptop.
- Run manually software updater.
- Software Updater indicated previous upgrade failed, proposed to "partially upgrade" --> confirmed yes.
- then Error popup message : "Can't install 'lubuntu-core'

It was impossible to install a required package. Please report this as a bug using 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal."
- Then proceeded to 'ubuntu-bug ubuntu-release-upgrader-core' via Terminal

lsb_release -rd
Description: Ubuntu 18.04.5 LTS
Release: 18.04

apt-cache policy ubuntu-release-upgrader-core
ubuntu-release-upgrader-core:
  Installed: 1:18.04.38
  Candidate: 1:20.04.25
  Version table:
     1:20.04.25 500
        500 http://us.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
        500 http://us.archive.ubuntu.com/ubuntu focal-updates/main i386 Packages
     1:20.04.18 500
        500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
        500 http://us.archive.ubuntu.com/ubuntu focal/main i386 Packages
 *** 1:18.04.38 100
        100 /var/lib/dpkg/status

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: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: LXDE
Date: Sat Oct 3 17:39:27 2020
InstallationDate: Installed on 2018-09-11 (753 days ago)
InstallationMedia: Lubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDistupgradeTermlog:

[Launchpad Janitor (janitor)]

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