unable to upgrade to new release

[Riccardo]

Got an error while trying upgrading.
Just trying to run the upgrade using the graphical UI.
Hopefully the required logs have been collected authomatically by the ubuntu-bug utility.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.19
ProcVersionSignature: Ubuntu 3.2.0-90.128-generic 3.2.69
Uname: Linux 3.2.0-90-generic i686
ApportVersion: 2.0.1-0ubuntu17.9
Architecture: i386
Date: Sat Sep 5 19:31:56 2015
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1441473907
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 1026
 com.ubuntu.update-manager window-width 1615
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
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 2015-09-05 (0 days ago)
VarLogDistupgradeTermlog: