Upgrade from 18.04 to 20.04 is incalculable due to python-qt4-dev being installed

[Xavier Andreu Álvarez]

The update could not be calculated

An unresolved issue occurred while calculating the update.

 This may be due to:
 * Unofficial software packages not provided by Ubuntu.
Use the "ppa-purge" tool in the ppa-purge package
to remove software from a Launchpad PPA and
tone it down to try.

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, the log files in "/ var / log / dist-upgrade" contain the details of the update. Specifically, see "main.log" and "apt.log."

I used "ppa-purge" tool, but it did not work.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 5.4.0-42.46~18.04.1-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: XFCE
Date: Wed Aug 12 20:31:41 2020
InstallationDate: Installed on 2019-08-17 (361 days ago)
InstallationMedia: Xubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-08-12 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2020-08-12T19:17:45.369468

[Launchpad Janitor (janitor)]

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