Upgrade from 12.04 to 14.04 failed

[XXLRay]

I get the following error message:

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.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.13
ProcVersionSignature: Ubuntu 3.2.0-61.93-generic 3.2.55
Uname: Linux 3.2.0-61-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Tue May 13 13:37:02 2014
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
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 2014-05-13 (0 days ago)
VarLogDistupgradeTermlog:

[Brian Murray (brian-murray)]

This seems to be due to some python packages which you've installed from a PPA.

libpython3.3 3.3.0-1irie1~precise1 1
libpython3.3-minimal 3.3.0-1irie1~precise1 1
libpython3.3-stdlib 3.3.0-1irie1~precise1 1
libpython3.4 3.4.0-2ubuntu1irie1~precise1 0
libpython3.4-minimal 3.4.0-2ubuntu1irie1~precise1 1
libpython3.4-stdlib 3.4.0-2ubuntu1irie1~precise1 1

You'll need to install the official package versions of any packages from that PPA for the upgrade to proceed successfully.

[XXLRay (xxlray)]

I could work around the problem by creating a 14.04 sources.list with the Ubuntu sources.list creator and executing sudo apt-get update && sudo apt-get dist-upgrade. Repeatingly executing sudo apt-get -f install && sudo apt-get dist-upgrade finally got me rid of any errors. So far the system seems to be running.