Ubuntu upgrade 18.04.5 lts to 21.04

[Georg Næumann]

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'.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.44
ProcVersionSignature: Ubuntu 4.15.0-151.157-generic 4.15.18
Uname: Linux 4.15.0-151-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.24
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Mon Aug 2 18:15:48 2021
InstallationDate: Installed on 2014-02-01 (2739 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-08-02 (0 days ago)