release-update tp 20.04 LTS failed

[U. Partheil]

1)
Description: Ubuntu 18.04.5 LTS
Release: 18.04

3) upgrade to 20.04 LTS
4)
Es konnte nicht ermittelt werden, welche Systemaktualisierungen verfügbar sind

Ein unlösbares Problem trat während der Berechnung der Systemaktualisierung auf.

 Wenn keine dieser zutrifft, melden Sie bitte den Fehler mithilfe des Befehls 'ubuntu-bug ubuntu-release-upgrader-core' in einem Terminal. Wenn Sie dies selbst untersuchen möchten, finden Sie in den Protokolldateien unter »/var/log/dist-upgrade« Details zur Systemaktualisierung. Schauen Sie sich insbesondere »main.log« und »apt.log« an.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-121.123-generic 4.15.18
Uname: Linux 4.15.0-121-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sun Oct 18 15:53:24 2020
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-18 (0 days ago)
VarLogDistupgradeTermlog:

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[U. Partheil (uptux)]

2021-08-23:
From time to time I checked upgrade-possibilities: always the same.
But last week the upgrade-process went further on -- until to a point, where it says something like: "upgrade only partially possible; continue? or continue?"
After that it was no longer possible to boot, I didn't even manage to reach grub
==> I gave up
and installed 20.04 from scratch.