submenus of the network menu do not work

When I click on the network menu it shows the usual behaviour, but the submenus 'More networks' or 'VPN connections' are empty, only a thin black bar appears behind the network menu. A click on the other buttons in the network menu yields no response.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: unity 5.18.0-0ubuntu2
ProcVersionSignature: Ubuntu 3.2.0-41.66-generic-pae 3.2.42
Uname: Linux 3.2.0-41-generic-pae i686
ApportVersion: 2.0.1-0ubuntu17.2
Architecture: i386
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,unitymtgrabhandles,workarounds,scale,expo,ezoom,unityshell]
Date: Sat May 4 16:03:32 2013
MarkForUpload: True
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)