failed to upgrade to 16.04

[[e-mail]]

I tried to upgrade 14.04 LTS to 14.10 and it failed with

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done

Calculating the changes

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.19.0-49.55~14.04.1-generic 3.19.8-ckt12
Uname: Linux 3.19.0-49-generic x86_64
NonfreeKernelModules: prl_fs_freeze prl_fs prl_eth prl_tg
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Thu Feb 4 11:18:04 2016
InstallationDate: Installed on 2016-01-28 (6 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-02-04 (0 days ago)
mtime.conffile..etc.update.manager.release.upgrades: 2016-02-04T11:15:00.391736

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.