'Err Upgrade tool' with 'Authentication failed'

I'm trying yo upgrade 18.04LTS to 20.04LTS.

What I've done in preparation:

• Comparing my sources.list with this (identical except us→nl subdomains)
• sudo apt update
• sudo apt install -y update-manager-core
• sudo apt-get --with-new-pkgs upgrade
• sudo apt remove wine-stable (package with unmet dependencies)
• sudo apt autoremove
• sudo apt-get dist-upgrade (didn't perform anything)
• sudo apt-get upgrade (because people say either apt or apt-get does more than the other)
• Reboot

Then its giving me this error:

mark@mark:~$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1554 B]                                                                                                                                                                      
Get:2 Upgrade tool [1342 kB]                                                                                                                                                                               
Err Upgrade tool                                                                                                                                                                                           
  Undetermined Error [IP: 213.136.12.213 80]                                                                                                                                                               
Fetched 1554 B 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. 

Searching with some of the key terms above gives 0 results on google.

There's no problem with my network, and there's plenty free disk space.

I tried removing /etc/apt/trusted.gpg, re-adding official Ubuntu keys, and doing apt-update. This succeeded, but it didn't help with the error above.