"An unresolvable problem occurred while calculating the upgrade." while upgrading from 18.04 to 20.04.1

[jordix]

Trying to upgrade from 18.04 to 20.04.1, I get the message:
"
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'.
"

I used the ubuntu-bug tool to report this, I hope all the needed files are attached :)
Thanks

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-112.113-generic 4.15.18
Uname: Linux 4.15.0-112-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu Aug 13 11:27:24 2020
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-08-13 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2020-08-13T10:50:11.930876

[Launchpad Janitor (janitor)]

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