Upgrade from 12.04 to 14.04 fails.

[Vedran Vyroubal]

Tried to perform an upgrade from 12.04 to 14.04, but it fails during the "setting new software channels" phase.
The error message is:
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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.
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All of the third-party sources have been disabled.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.13
ProcVersionSignature: Ubuntu 3.2.0-60.91-generic 3.2.55
Uname: Linux 3.2.0-60-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Tue Apr 22 14:25:18 2014
EcryptfsInUse: Yes
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1398167869
 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 LTS "Precise Pangolin" - Release amd64 (20120425)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-04-22 (0 days ago)
VarLogDistupgradeTermlog: