Upgrade from Ubuntu Mate 18.04.5 -> 20.04 via "Software Updater" or what is called by it later.
At first, calculation failed with reported suspicion on unofficial Ubuntu repositories (Launchpad). I've removed those via "Software&Updates". I've cleaned up sources.list.d from those, because upgrader had once restored repositories at "Other Software" tab after next failed upgrade. I've deleted keys for those repositories. I can't tell, if the latter was mandatory.
Upgrader hadn't stop failing. Same suspicion. I've switched off all repositories listed at "Other Software" tab. Now upgrader fails with no hint.
Description: Ubuntu 18.04.5 LTS
Release: 18.04
$ apt-cache policy ubuntu-release-upgrader
N: Unable to locate package ubuntu-release-upgrader
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-117.118-generic 4.15.18
Uname: Linux 4.15.0-117-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: MATE
Date: Wed Sep 9 10:59:07 2020
InstallationDate: Installed on 2016-10-27 (1412 days ago)
InstallationMedia: Ubuntu-MATE 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-09-09 (0 days ago)
VarLogDistupgradeTermlog:
mtime.conffile..etc.update-manager.release-upgrades: 2020-09-09T09:18:30.128932
Sorry, I had to finish failed upgrader before sending attachments.