Upgrade from 18.04.4 LTS to 20.04.1 LTS has failed on many occasions

[Sanyi64]

I tried to upgrade my Ubuntu system from 18.04.4 LTS to 20.04.1 LTS but the upgrade has failed.

The upgrade process always ending in error:

An unresolvable problem occurred while calculating the upgrade.

 If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If you want to investigate this yourself the log files in '/var/log/dist-upgrade' will contain details about the upgrade. Specifically, look at 'main.log' and 'apt.log'.

Description: Ubuntu 18.04.4 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37 [origin: unknown]
ProcVersionSignature: Ubuntu 4.15.0-101.102-generic 4.15.18
Uname: Linux 4.15.0-101-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sat Nov 21 22:04:47 2020
InstallationDate: Installed on 2018-08-26 (818 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=hu_HU.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-11-21 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

In your /etc/apt/soures.list file all references to the official Ubuntu archive have been commented out. You'll need to resolve this before trying the upgrade again. Here's part of what it looks like:

# deb http://hu.old-releases.ubuntu.com/ubuntu/ bionic multiverse
# deb-src http://hu.old-releases.ubuntu.com/ubuntu/ bionic multiverse
# deb http://hu.old-releases.ubuntu.com/ubuntu/ bionic-updates multiverse
# deb-src http://hu.old-releases.ubuntu.com/ubuntu/ bionic-updates multiverse

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]