can not upgrade to release 18.04 from 16.04

[Oleg Bahin]

Upgrading to release 18.04 crashes without any meaningful error, only

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.4.0-112.135-generic 4.4.98
Uname: Linux 4.4.0-112-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: XFCE
Date: Fri Dec 7 13:28:02 2018
InstallationDate: Installed on 2016-05-18 (932 days ago)
InstallationMedia: Xubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-12-07 (0 days ago)

[Juhani Numminen (jsonic)]

Hi!

It seems like you've installed 3rd party drivers. Please remove them and see if the upgrade succeeds then.

Quoting https://launchpad.net/~oibaf/+archive/ubuntu/graphics-drivers

=== Upgrading to a newer Ubuntu ===
It is *strongly* suggested to remove all packages from this PPA before updating to a newer Ubuntu release. See the section "Revert to original drivers" later on.
Then, after the upgrade, you can add again this PPA.

=== Revert to original drivers ===
To revert to standard Ubuntu drivers type the following in a prompt shell:
$ sudo apt-get install ppa-purge
$ sudo ppa-purge ppa:oibaf/graphics-drivers

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]