upgrade not possible. the exact problem isn't shown

[Christina]

"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 1:18.04.42
ProcVersionSignature: Ubuntu 5.4.0-60.67~18.04.1-generic 5.4.78
Uname: Linux 5.4.0-60-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: X-Cinnamon
Date: Mon Jan 11 19:53:40 2021
InstallationDate: Installed on 2019-04-17 (635 days ago)
InstallationMedia: Ubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
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 2021-01-11 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

Thanks for taking the time to report this upgrade failure. There is currently a new version of the release upgrader availalbe in -proposed which may resolve the error you've encountered. Please run the following command to test the new version of the upgrader:

  do-release-upgrade -p

Please lets us know the results in this bug report. Thanks again and good luck!

[Launchpad Janitor (janitor)]

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