I have the same issue and it does not just happen on Kubuntu only installs. I installed Ubuntu and then KDE 4 and I get the same problem when using Synaptic in KDE 4. My Nvidia graphics settings got messed up somehow and I had to do a fresh reinstall and still get this error when using Synaptic in KDE 4. It seems to finish the install and just hangs there.
I have the same issue and it does not just happen on Kubuntu only installs. I installed Ubuntu and then KDE 4 and I get the same problem when using Synaptic in KDE 4. My Nvidia graphics settings got messed up somehow and I had to do a fresh reinstall and still get this error when using Synaptic in KDE 4. It seems to finish the install and just hangs there.