do-release-upgrade from 18.04 aborting

Question

• main.log

2024-03-01 14:10:09,360 INFO Using config files '['./DistUpgrade.cfg.bionic', '/etc/update-manager/release-upgrades.d/ubuntu-advantage-upgrades.cfg']'
2024-03-01 14:10:09,360 INFO uname information: 'Linux pruttel 4.15.0-213-generic #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 x86_64'
2024-03-01 14:10:09,891 INFO apt version: '1.6.17'
2024-03-01 14:10:09,892 INFO python version: '3.6.9 (default, Mar 10 2023, 16:46:00) 
[GCC 8.4.0]'
2024-03-01 14:10:09,896 INFO release-upgrader version '20.04.40' started
2024-03-01 14:10:09,911 INFO locale: 'en_US' 'UTF-8'
2024-03-01 14:10:09,976 INFO screen could not be run
2024-03-01 14:10:10,017 DEBUG Using 'DistUpgradeViewText' view
2024-03-01 14:10:10,075 DEBUG enable dpkg --force-overwrite
2024-03-01 14:10:10,126 DEBUG creating statefile: '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'
2024-03-01 14:10:15,862 DEBUG lsb-release: 'bionic'
2024-03-01 14:10:15,863 DEBUG _pythonSymlinkCheck run
2024-03-01 14:10:15,867 DEBUG openCache()
2024-03-01 14:10:15,867 DEBUG quirks: running PreCacheOpen
2024-03-01 14:10:15,867 DEBUG running Quirks.PreCacheOpen
2024-03-01 14:10:17,460 DEBUG Comparing 4.15.0-212 with 
2024-03-01 14:10:17,460 DEBUG Comparing 4.15.0-213 with 4.15.0-212
2024-03-01 14:10:18,030 DEBUG /openCache(), new cache size 116775
2024-03-01 14:10:18,030 DEBUG need_server_mode(): run in 'desktop' mode, (because of key deps for 'ubuntu-desktop')
2024-03-01 14:10:18,030 DEBUG checkViewDepends()
2024-03-01 14:10:18,035 DEBUG running doUpdate() (showErrors=False)
2024-03-01 14:10:20,584 DEBUG openCache()
2024-03-01 14:10:22,211 DEBUG Comparing 4.15.0-212 with 
2024-03-01 14:10:22,211 DEBUG Comparing 4.15.0-213 with 4.15.0-212
2024-03-01 14:10:22,802 DEBUG /openCache(), new cache size 116775
2024-03-01 14:10:22,803 DEBUG doPostInitialUpdate
2024-03-01 14:10:22,803 DEBUG quirks: running focalPostInitialUpdate
2024-03-01 14:10:22,803 DEBUG running Quirks.focalPostInitialUpdate
2024-03-01 14:10:25,242 DEBUG abort called
2024-03-01 14:10:25,242 DEBUG openCache()
2024-03-01 14:10:26,879 DEBUG Comparing 4.15.0-212 with 
2024-03-01 14:10:26,879 DEBUG Comparing 4.15.0-213 with 4.15.0-212
2024-03-01 14:10:27,452 DEBUG /openCache(), new cache size 116775

• apt.log

Log time: 2024-03-01 14:10:18.023495
Log time: 2024-03-01 14:10:22.784019
Log time: 2024-03-01 14:10:27.445820

Upgrading from 18.04 LTS to 20.04 LTS gives the following output:

$ sudo do-release-upgrade 
[sudo] password for marijn: 
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1554 B]                                          
Get:2 Upgrade tool [1338 kB]                                                   
Fetched 1340 kB in 0s (0 B/s)                                                  
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg' 
extracting 'focal.tar.gz'
Reading cache
Checking package manager
Reading package lists... Done
Building dependency tree

Reading state information... Done
Hit http://nl.archive.ubuntu.com/ubuntu bionic InRelease

Hit http://nl.archive.ubuntu.com/ubuntu bionic-updates InRelease

Hit http://nl.archive.ubuntu.com/ubuntu bionic-security InRelease

Hit http://nl.archive.ubuntu.com/ubuntu bionic-backports InRelease

Hit https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease

Fetched 0 B in 0s (0 B/s)

Reading package lists... Done

Building dependency tree

Reading state information... Done
Restoring original system state
Aborting
Reading package lists... Done

Building dependency tree

Reading state information... Done
$

The log files (see above) also give no indication why the process is aborting. The system is up-to-date (within 18.04 at least). How do I figure out what the problem is?