Comment 8 for bug 721738

@andrews: My '/var/lib/dpkg/available' was fine, so this was not a solution. I was able to downgrade debconf since I still had the Maverick repositories in my sources, so I could downgrade the package via Synaptic.

I have managed to solve the problem by commenting out lines 26-28 in file '/usr/share/perl5/Debconf/FrontEnd/Gnome.pm'.

The lines now look like this:
# if ($this->logo) {
# $this->assistant->set_page_header_image($this->assistant_page, $this->logo);
# }

Everything seems fine now, no more terminal errors while installing or uninstalling certain packages via Synaptic. I cannot guarantee that this is safe though.