Upgrade from Lubuntu 18.04 to 20.04 not possible

[Isving Wolfsdottir]

Ubuntu-Release:
Description: Ubuntu 18.04.5 LTS
Release: 18.04

Upgrade to 20.04 doesn't work

"do-release-upgrade" crashes already with:
----------------
It could not be determined which system updates are available

An unsolvable problem occurred during the calculation of System Update to.

This was caused by:
* System update to a pre-release version of Ubuntu
This is most likely a temporary problem, please try again later.
If none of these apply, please report the error using the
Command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.
If you want to investigate this yourself, you will find a description in the
log files under "/var/log/dist-upgrade" Details on
System update. Look especially at "main.log" and
"apt.log" on.
---------------

I've already tried without success to remove 3rd party package sources like Steam and WireGuard and removed the installed programs.

Unfortunately I haven't also additional information what happened.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.40
ProcVersionSignature: Ubuntu 4.15.0-121.123-generic 4.15.18
Uname: Linux 4.15.0-121-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
Date: Mon Oct 19 01:04:02 2020
InstallationDate: Installed on 2019-01-01 (656 days ago)
InstallationMedia: Lubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=de_DE
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-18 (0 days ago)
VarLogDistupgradeTermlog:

[Chris Guiver (guiverc)]

The Lubuntu 18.04 to Lubuntu 20.04 upgrade path is not supported by the Lubuntu team because of the switch of desktop. If you refer to the release notes of any release starting with 18.10 you'll note

"Note, due to the extensive changes required for the shift in desktop environments, the Lubuntu team does not support upgrading from 18.04 or below to any greater release. Doing so will result in a broken system. If you are on 18.04 or below and would like to upgrade, please do a fresh install."

https://lubuntu.me/focal-1-released/

[Launchpad Janitor (janitor)]

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