ubuntu-bug ubuntu-release-upgrader-core

[EmilDo]

When I try to upgrade from Ub18LTS to Ub20LTS, it breaks down because of inofficial software package.
However I do not know which package and how to remove it.
The proposed files »main.log« und »apt.log« I can not interpret.

Description: Ubuntu 18.04.5 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-122.124-generic 4.15.18
Uname: Linux 4.15.0-122-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 23 22:11:12 2020
InstallationDate: Installed on 2018-08-27 (788 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=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-23 (0 days ago)