Upgrade from Ubuntu 18.04.5 LTS to Ubuntu 20.04.1 LTS failed - unresolvable problem

[Vladimir Petrov]

I used the default GUI software updater which suggested me to update to Ubuntu 20. Then the following error was shown:

---
Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This was likely caused by:
 * Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

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'.
---

The following command did not produce any output:
sudo grep -i ppa.launchpad.net /etc/apt/sources.list.d/*.list

I read the logs in '/var/log/dist-upgrade' but couldn't find anything useful.

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
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 9 22:26:24 2020
InstallationDate: Installed on 2020-02-23 (229 days ago)
InstallationMedia: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-09 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1898152, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.