Upgrade from 18.04 to 20.04 leaves system unusable

[Juergen Key]

Update process ends prematurely, after apt --fix-broken install - it answers:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Correcting dependencies... failed.
The following packages have unmet dependencies:
 libc-bin : Depends: libc6 (< 2.28) but 2.31-0ubuntu9.1 is installed
 libc-dev-bin : Depends: libc6 (< 2.28) but 2.31-0ubuntu9.1 is installed
 libc6 : Recommends: libidn2-0 (>= 2.0.5~) but 2.0.4-1.1ubuntu0.2 is installed
         Breaks: libc6:i386 (!= 2.31-0ubuntu9.1) but 2.27-3ubuntu1.2 is installed
 libc6:i386 : Breaks: libc6 (!= 2.27-3ubuntu1.2) but 2.31-0ubuntu9.1 is installed
 libc6-dbg : Depends: libc6 (= 2.27-3ubuntu1.2) but 2.31-0ubuntu9.1 is installed
 libc6-dev : Depends: libc6 (= 2.27-3ubuntu1.2) but 2.31-0ubuntu9.1 is installed
 libc6-i386 : Depends: libc6 (= 2.27-3ubuntu1.2) but 2.31-0ubuntu9.1 is installed
 libcrypt1 : Breaks: libc6:i386 (< 2.31) but 2.27-3ubuntu1.2 is installed
 libp11-kit0:i386 : Depends: libc6:i386 (>= 2.28) but 2.27-3ubuntu1.2 is installed
 locales : Depends: libc-bin (> 2.31)
 p11-kit-modules : Depends: libp11-kit0 (= 0.23.9-2) but 0.23.20-1build1 is installed
 python-gobject-2-dev : Depends: python-dev
 python-is-python2 : Depends: python2 but it is not installed
 python-opengl : Depends: python-ctypes
 python-samba : Depends: python-tdb but it is not installable
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.
E: Unable to correct dependencies

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-118.119-generic 4.15.18
Uname: Linux 4.15.0-118-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
Date: Fri Oct 9 18:09:21 2020
InstallationDate: Installed on 2012-03-02 (3142 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Alpha amd64 (20120302)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=screen
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-09 (0 days ago)
VarLogDistupgradeXorgFixuplog:
 INFO:root:/usr/bin/do-release-upgrade running
 INFO:root:No xorg.conf, exiting