Precise->Trusty upgrade calculation fails

[Aurélien COUDERC]

Up to date 12.04.4 (precise)
Tried to upgrade to 14.04 (trusty) with "update-manager -d" but the resolution of the upgrade fails.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.15
ProcVersionSignature: Ubuntu 3.13.0-30.55~precise1-generic 3.13.11.2
Uname: Linux 3.13.0-30-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Sun Jul 13 12:03:43 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1405245714
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 672
 com.ubuntu.update-manager window-width 600
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-07-13 (0 days ago)
VarLogDistupgradeTermlog: