The system can not be upgraded to version 18.04.1

[Jacek]

Ubuntu 16.04.5 LTS (LXDE).

When trying to update the system online from 16/04 to 18/04, the following error will be issued:

An unresolved issue occurred during the upgrade.

It can be caused by:
* Update to the test version of Ubuntu
* Operation on the trial version of Ubuntu
* Unofficial software packages not provided by Ubuntu

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.4.0-135.161-generic 4.4.140
Uname: Linux 4.4.0-135-generic i686
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: LXDE
Date: Thu Sep 20 10:11:20 2018
InstallationDate: Installed on 2017-12-13 (280 days ago)
InstallationMedia: Lubuntu 16.04.1 LTS "Xenial Xerus" - Release i386 (20160720)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-09-20 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

From VarLogDistupgradeMainlog.txt:

2018-09-20 10:10:57,848 DEBUG Marking 'lubuntu-core' for upgrade
2018-09-20 10:10:58,097 DEBUG blacklist expr 'update-manager$' matches 'update-manager'
2018-09-20 10:10:58,097 DEBUG The package 'update-manager' is marked for removal but it's in the removal blacklist