Start of LTS upgrade 18.04 to 20.04 failed/aborted

[Olaf Encke]

do-release-upgrade aborted with error:

"Es konnte nicht ermittelt werden, welche Systemaktualisierungen
verfügbar sind
Ein unlösbares Problem trat während der Berechnung der
Systemaktualisierung auf.
Wenn keine dieser zutrifft, melden Sie bitte den Fehler mithilfe des
Befehls 'ubuntu-bug ubuntu-release-upgrader-core' in einem Terminal.
Wenn Sie dies selbst untersuchen möchten, finden Sie in den
Protokolldateien unter »/var/log/dist-upgrade« Details zur
Systemaktualisierung. Schauen Sie sich insbesondere »main.log« und
»apt.log« an. "

Before that it aborted and told to use ppa-purge, but not which package to purge, so I removed all launchpad/ppa sources from the source list.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.42
ProcVersionSignature: Ubuntu 4.15.0-136.140-generic 4.15.18
Uname: Linux 4.15.0-136-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Feb 25 22:57:24 2021
InstallationDate: Installed on 2016-04-28 (1764 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-02-25 (0 days ago)
VarLogDistupgradeTermlog:

[Olaf Encke (olaf-encke)]

How is it possible to identfy the key package(s), that hinder(s) the upgrade?

[Olaf Encke (olaf-encke)]

Ah, kisak-mesa seems to have been the culprit.
After
sudo ppa-purge ppa:kisak/kisak-mesa
the upgrade is now running.

[Brian Murray (brian-murray)]

Reviewing the files from your upgrade attempt you seem to have enabled a PPA that provides Xorg packages. This will prevent the upgrade as that PPA contains package version numbers greater than the release to which you are upgrading. You can revert the PPA by installing the ppa-purge package from the official Ubuntu archive and then running 'sudo ppa-purge ppa:oibaf/graphics-drivers'. After that you can try upgrading again and if it still fails open a new bug report. Thanks and good luck!