do_release_upgrade fails from bionic to focal, with "authentication failed"

[[e-mail]]

I want to upgrade from 18.04.1 to 20.04.1

sudo do-release-upgrade
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1,554 B]
Get:2 Upgrade tool [1,342 kB]
Fetched 1,344 kB in 0s (0 B/s)
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.

I have tried setting my download source to "Main Servers" (was GB) - no difference.

I have tried "apt install --reinstall ubuntu-keyring" - no difference.

I had already performed an apt upgrade before I started. I've tried apt upgrade again - nothing to upgrade.

Thanks for any help with this.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.17
ProcVersionSignature: Ubuntu 4.15.0-118.119-generic 4.15.18
Uname: Linux 4.15.0-118-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.16
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sat Oct 3 10:59:35 2020
EcryptfsInUse: Yes
InstallationDate: Installed on 2016-11-27 (1405 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2018-11-25 (677 days ago)

[[e-mail] (tenminjoe)]

I ran

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3B4FE6ACC0B21F32

and

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 871920D1991BC93C

and that seems to have fixed it