Comment 5 for bug 270626

I've edited /var/cache/debconf/config.dat manually because "sudo dpkg-reconfigure debconf" didn't work anymore, too. I've changed
   Name: debconf/frontend
   Template: debconf/frontend
   Value: Kde
   Owners: debconf
   Flags: seen
to
   [...]
   Value: Dialog
   [...]

That worked, so the problem seems to be in libqt-perl or the qt-frontend of debconf.