No se puede actualizar de 16.04 a 18.04, ¿no hay un error real?

Recibí el aviso de que hay una actualización para mi instalación de escritorio, pero no puedo hacerlo. No hay mensajes de error reales que pueda ver. He corrido sudo apt autoremove, sudo apt upgradey sudo apt install, y da 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. Esta es mi pantalla cuando ejecuto 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 es mi 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

Cual podría ser el problema aquí?

Editar

Usando el enlace proporcionado por WinEunuuchs2Unix, aquí están mis enlaces 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'

tratar:

sudo apt-get install lsb-core

y luego actualizar

A veces este paquete falta o está dañado y el instalador tiene dificultades para determinar qué versión se está ejecutando actualmente. Hay muchas quejas sobre eso si lo buscas en Google. Lo mismo podría suceder en Debian.

Ubuntu ha dejado de admitir sistemas de escritorio de 32 bits a partir de 17.10. La última versión de 32 bits es 16.04.5, por https://www.ubuntu.com/download/alternative-downloads .

wow que ciego estoy.

Echo de menos la fuente del paquete xenial-updates.

Expande tu /etc/apt/sources.list

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

Actualizar xenial

sudo apt update && sudo apt full-upgrade

Vuelva a intentarlo. Si esto no funciona, me quedo sin ideas.

Y sabes copia de seguridad 1.