upgrade from 18.04 to 20.04 fails

[Tomasz Binkiewicz]

Couldn't upgrade from Ubuntu 18.04 to 20.04.
Error message:
'''
  Could not calculate the upgrade

  An unresolvable problem occurred while calculating the upgrade.

   This was likely caused by:
   * Unofficial software packages not provided by Ubuntu
  Please use the tool 'ppa-purge' from the ppa-purge
  package to remove software from a Launchpad PPA and
  try the upgrade again.

  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'.
'''
I tried to remove PPA packages, but couldn't found any.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 5.4.0-48.52~18.04.1-generic 5.4.60
Uname: Linux 5.4.0-48-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Oct 8 20:15:32 2020
InstallationDate: Installed on 2019-09-16 (387 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-08 (0 days ago)
VarLogDistupgradeTermlog:

[Daniel Letzeisen (dtl131)]

https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1889250/comments/2