Upgrade from 18.04 to 20.04 fails

[Oleg "Nightwing" Lomakin]

ubuntu-release-upgrader can't upgrade my system from bionic to focal because of "held" packages, apt-mark showhold shows nothing.

I started upgrade process from update-manager (launched from gnome menu, without '-d' option), but got error:

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'.

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: Tue Sep 29 14:59:03 2020
InstallationDate: Installed on 2018-12-03 (665 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=ru_RU.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-09-29 (0 days ago)
VarLogDistupgradeTermlog: