Update-manager won't upgrade from 12.04 with unclear error

[Martijn Lasthuizen]

When updating from 12.04 desktop update-manager stops when at the fase of really downloading new packages.
It is unclear what causes the error or what to do to make it upgrade to 14.04

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: nvidia
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Thu Apr 24 09:54:52 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1398151944
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 615
 com.ubuntu.update-manager window-width 600
MarkForUpload: True
PackageArchitecture: all
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-04-24 (0 days ago)
VarLogDistupgradeTermlog:

[Martijn Lasthuizen (m-lasthuizen)]

dist-upgrade