Upgrade from 18 to 20 fails while calculating the upgrade

[Ilektra]

It first failed because of non-ubuntu ppa's. Now they're all removed (unticked in the updater options), it still fails with no explanation:

Could not determine the upgrade

An unresolvable problem occurred while calculating the upgrade.

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'.

Description: Ubuntu 18.04.5 LTS
Release: 18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.42
ProcVersionSignature: Ubuntu 4.15.0-140.144-generic 4.15.18
Uname: Linux 4.15.0-140-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Tue Apr 6 14:42:49 2021
InstallationDate: Installed on 2018-11-01 (886 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2021-04-06 (0 days ago)
VarLogDistupgradeTermlog: