Não foi possível atualizar de 16.04 para 18.04, nenhum erro real?

Recebi o aviso de que há uma atualização para minha instalação da área de trabalho, mas não consigo fazer isso. Não há mensagens de erro reais que eu possa ver. Já corri sudo apt autoremove, sudo apt upgradee sudo apt install, e dá 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. Esta é a minha tela quando executo do-release-upgrade:

Reading cache

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Hit http://us.archive.ubuntu.com/ubuntu xenial InRelease
Hit http://archive.canonical.com xenial InRelease
Hit http://security.ubuntu.com/ubuntu xenial-security 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
=== Command detached from window (Mon Aug 20 20:34:56 2018) ===

Este é o meu main.log:

2018-08-20 20:34:50,649 INFO Using config files '['./DistUpgrade.cfg.xenial']'
2018-08-20 20:34:50,649 INFO uname information: 'Linux egwene 4.4.0-133-generic #159-Ubuntu SMP Fri Aug 10 07:26:31 UTC 2018 i686'
2018-08-20 20:34:50,862 INFO apt version: '1.2.15ubuntu0.2'
2018-08-20 20:34:50,862 INFO python version: '3.5.2 (default, Nov 23 2017, 16:37:01)
[GCC 5.4.0 20160609]'
2018-08-20 20:34:50,864 INFO release-upgrader version '18.04.24' started
2018-08-20 20:34:50,868 INFO locale: 'en_US' 'UTF-8'
2018-08-20 20:34:51,059 DEBUG Using 'DistUpgradeViewText' view
2018-08-20 20:34:51,111 DEBUG enable dpkg --force-overwrite
2018-08-20 20:34:51,150 DEBUG creating statefile: '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'
2018-08-20 20:34:54,329 DEBUG lsb-release: 'xenial'
2018-08-20 20:34:54,330 DEBUG _pythonSymlinkCheck run
2018-08-20 20:34:54,331 DEBUG openCache()
2018-08-20 20:34:54,332 DEBUG No such plugin directory: ./plugins
2018-08-20 20:34:54,332 DEBUG plugins for condition 'PreCacheOpen' are '[]'
2018-08-20 20:34:54,332 DEBUG plugins for condition 'bionicPreCacheOpen' are '[]'
2018-08-20 20:34:54,332 DEBUG plugins for condition 'from_xenialPreCacheOpen' are '[]'
2018-08-20 20:34:54,333 DEBUG quirks: running PreCacheOpen
2018-08-20 20:34:54,333 DEBUG running Quirks.PreCacheOpen
2018-08-20 20:34:54,721 DEBUG /openCache(), new cache size 56038
2018-08-20 20:34:54,721 DEBUG need_server_mode(): run in 'desktop' mode, (because of pkg 'ubuntu-desktop')
2018-08-20 20:34:54,721 DEBUG checkViewDepends()
2018-08-20 20:34:54,721 DEBUG running doUpdate() (showErrors=False)
2018-08-20 20:34:55,529 DEBUG openCache()
2018-08-20 20:34:55,939 DEBUG /openCache(), new cache size 56038
2018-08-20 20:34:55,940 DEBUG doPostInitialUpdate
2018-08-20 20:34:55,940 DEBUG plugins for condition 'PostInitialUpdate' are '[]'
2018-08-20 20:34:55,940 DEBUG plugins for condition 'bionicPostInitialUpdate' are '[]'
2018-08-20 20:34:55,940 DEBUG plugins for condition 'from_xenialPostInitialUpdate' are '[]'
2018-08-20 20:34:55,940 DEBUG quirks: running bionicPostInitialUpdate
2018-08-20 20:34:55,940 DEBUG running Quirks.bionicPostInitialUpdate
2018-08-20 20:34:55,951 DEBUG abort called
2018-08-20 20:34:55,951 DEBUG openCache()
2018-08-20 20:34:55,951 DEBUG failed to SystemUnLock() (E:Not locked)
2018-08-20 20:34:56,356 DEBUG /openCache(), new cache size 56038

Qual poderia ser o problema aqui?

Editar

Usando o link fornecido pelo WinEunuuchs2Unix, aqui estão os meus links PPA:

'http://us.archive.ubuntu.com/ubuntu/ xenial main restricted'
'http://us.archive.ubuntu.com/ubuntu/ xenial universe'
'http://us.archive.ubuntu.com/ubuntu/ xenial multiverse'
'http://security.ubuntu.com/ubuntu xenial-security main restricted'
'http://security.ubuntu.com/ubuntu xenial-security universe'
'http://security.ubuntu.com/ubuntu xenial-security multiverse'
'http://archive.canonical.com/ xenial partner'

experimentar:

sudo apt-get install lsb-core

e depois atualizar

Às vezes, este pacote está ausente ou corrompido e o instalador tem dificuldade em descobrir qual versão está sendo executada atualmente. O mesmo pode acontecer no Debian.

O Ubuntu parou de suportar sistemas de desktop de 32 bits a partir de 17.10. A versão mais recente do 32bit é 16.04.5, por https://www.ubuntu.com/download/alternative-downloads .

uau, tão cego eu sou.

Sinto falta da fonte do pacote xenial-updates.

Expanda seu /etc/apt/sources.list

deb http://us.archive.ubuntu.com/ubuntu xenial-updates main restricted universe multiverse

Atualizar xenial

sudo apt update && sudo apt full-upgrade

Então tente novamente. Se isso não funcionar, estou sem ideias.

E você sabe o backup 1.