This error is not caused by Scribus but by the combination of Qt and Gtk
window theme. Normally Qt applications don't use Gtk themes, but Qt has a
special QGtkWindowStyle that reads the Gtk theme and tries to emulate that
in Qt. For this it uses the cairo graphics library, which is also used by
Scribus for other purposes. Somewhere along that road something goes wrong
and Scribus crashes.
Currently the only known workaround is to use qtconfig-qt4 to set the
appearance to something else, e.g. "Cleanlooks".
We Scribus developers can do little about this bug, since it is most likely
a version conflict between the Qt/gtk/cairo libraries.