Could not calculate the Upgrade from 12.04 to 14.04

[Patrick Doyle]

I tried upgrading from 12.04 to 14.04 and received a "Could not calculate the Upgrade" error message. As I am _not_

 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu

I am reporting this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal as suggested by the error message.

I _do_ note that I am likely to have "Unofficial software packages not provided by Ubuntu", but I would expect the upgrade process to be able to either work around that, or to provide me with additional information that would point me in the right direction.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.17
ProcVersionSignature: Ubuntu 3.2.0-67.101-generic 3.2.60
Uname: Linux 3.2.0-67-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Mon Sep 22 11:34:52 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1411399575
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 604
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 SHELL=/bin/bash
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-09-22 (0 days ago)
VarLogDistupgradeTermlog: