Upgrade 14.04 to 14.10 failed with ‘Could not determine the upgrade’

[Smylers]

Running Lubuntu 14.04, I tried to upgrade to 14.10 with:

  $ update-manager -d &

I pressed the ‘Upgrade’ button, then got a window with the release information, which started:

  Welcome to the Ubuntu 'Utopic Unicorn' development release

  This release is still in development. Do not install it on production machines.

That isn't right (it has now been released), but I proceeded anyway.

  Could not determine 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.

None of the above apply, hence this report.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.5
ProcVersionSignature: Ubuntu 3.13.0-41.70-generic 3.13.11.11
Uname: Linux 3.13.0-41-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.6
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: LXDE
Date: Wed Dec 3 13:16:58 2014
InstallationDate: Installed on 2012-03-01 (1007 days ago)
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2014-12-03 (0 days ago)
VarLogDistupgradeTermlog: