cannot upgrade to focal fossa

[carlo giovanardi]

the message from the upgrader is:
----------------------------------------
Could not calculate the upgrade

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'.
----------------------------------------

looked at the 2 log files above but did not find anything
I could intervene on

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: Wed Oct 14 09:44:33 2020
InstallationDate: Installed on 2019-09-24 (385 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-14 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

This should be resolved by the latest version of ubuntu-release-upgrader. Could you please try the upgrade again? The version is found in /var/log/dist-upgrade/main.log. You are looking for the following line "INFO release-upgrader version '20.04.25' started" and you want version '20.04.28'.

[Launchpad Janitor (janitor)]

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