Update-manager doesn't list available updates

[Jussi]

Update-manager will announce updates are available, but sometimes it fails to show what the updates are.
See screen-shot.

Update from terminal works:

$ sudo apt-get upgrade
[sudo] password for xxxxxxxxxxx:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be upgraded:
  xfce4-session
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 788 kB of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue [Y/n]?

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: update-manager 1:0.186.1
ProcVersionSignature: Ubuntu 3.8.0-29.42-generic 3.8.13.5
Uname: Linux 3.8.0-29-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.9.2-0ubuntu8.3
Aptdaemon:

Architecture: amd64
Date: Fri Aug 23 15:49:24 2013
ExecutablePath: /usr/bin/update-manager
GsettingsChanges:
 b'com.ubuntu.update-manager' b'first-run' b'false'
 b'com.ubuntu.update-manager' b'launch-time' b'1377261983'
 b'com.ubuntu.update-manager' b'show-details' b'true'
 b'com.ubuntu.update-manager' b'window-height' b'745'
 b'com.ubuntu.update-manager' b'window-width' b'694'
InstallationDate: Installed on 2010-12-17 (979 days ago)
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
InterpreterPath: /usr/bin/python3.3
MarkForUpload: True
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to raring on 2013-05-06 (108 days ago)