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.
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.