upgrade from 18.04.5 LTS to 20.4 LTS failed: Could not calculate the upgrade

[Sebastien DESERT]

automatic upgrade from 18.04.6 LTS to 20.04 LTS was done, but not successfull.
Error message "Could not calculate the upgrade"

last lines in /var/log/dist-upgrade$ cat main.log :

2021-05-20 08:11:42,504 INFO removing python-minimal because python-is-python2 is being installed
2021-05-20 08:11:42,504 DEBUG Removing 'python-minimal' (python-is-python2 is being installed on the system)
2021-05-20 08:11:42,504 INFO failed to remove python-minimal
2021-05-20 08:11:42,504 INFO checking for python-dev (auto_inst=True)
2021-05-20 08:11:42,504 INFO checking for libpython-dev (auto_inst=True)
2021-05-20 08:11:42,504 INFO checking for libpython-stdlib (auto_inst=True)
2021-05-20 08:11:42,504 INFO removing libpython-stdlib because None is being installed
2021-05-20 08:11:42,504 DEBUG Removing 'libpython-stdlib' (None is being installed on the system)
2021-05-20 08:11:42,504 INFO failed to remove libpython-stdlib
2021-05-20 08:11:42,505 INFO checking for libpython-dbg (auto_inst=True)

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.44
ProcVersionSignature: Ubuntu 4.15.0-143.147-generic 4.15.18
Uname: Linux 4.15.0-143-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu May 20 09:00:34 2021
InstallationDate: Installed on 2013-02-06 (3024 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-05-20 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

Thanks for taking the time to report this upgrade failure. There is currently a new version of the release upgrader available in -proposed which may resolve the error you've encountered. Please run the following command to test the new version of the upgrader:

  do-release-upgrade -p

Please lets us know the results in this bug report. Thanks again and good luck!

[Sebastien DESERT (sebdesrt)]

Hi Brian,
upgrade to 20.04.2 LTS done successfully with the proposed command.
This ticket can be closed then.
Thanks.