Upgrade error message (18.04 to 20.04)

[Louis Vittaut]

I'm on Ubuntu 18.04 and I wanted to upgrade to Ubuntu 20.04. But from the upgrade calculation, an error message occurred :

Upgrade error message (18.04 to 20.04): An unsolvable problem occurred during the upgrade calculation. This was probably caused by unofficial software packages that do not come from Ubuntu.

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: ubuntu:GNOME
Date: Sun Oct 11 19:29:03 2020
InstallationDate: Installed on 2018-08-05 (797 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-11 (0 days ago)