Update Manager can't perform 'check' because of repository issue

[TenLeftFingers]

Actual behaviour: A condition reported in a dialog box (see attachment) prevents update-manager from performing a check. This means I will not receive any further updates until this particular issue is resolved.

Expected behaviour: Whatever the problem is, it should not block further operation of the update-manager.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.11
ProcVersionSignature: Ubuntu 3.2.0-38.61-generic-pae 3.2.37
Uname: Linux 3.2.0-38-generic-pae i686
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu17.1
Architecture: i386
Date: Tue Feb 26 15:29:36 2013
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1361875327
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 1000
 com.ubuntu.update-manager window-width 1215
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Beta i386 (20120328)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: No upgrade log present (probably fresh install)

[TenLeftFingers (tenleftfingers)]

Update: It seems that updates are coming through, but as a result of this issue the application incorrectly reports that my system was last updated 6 days ago.