the GTK# 2.12.40-1 library used by Smuxi and other C# GTK+ applications introduced a regression to leads to a NullReferenceException or Glib/GTK CRITICAL messages or SEGV.
For more about this RC bug details see #860450
I have backported only the fix that addresses the mentioned bug above in gtk-sharp2 2.12.40-2, no other changes.
Please review my attached debdiff of 2.12.40-1.1 (which is currently in testing) and 2.12.40-2 (which is currently in unstable):
debdiff gtk-sharp2_2.12.40-1.1.dsc ../build-area/gtk-sharp2_2.12.40-2.dsc > debdiff
unblock gtk-sharp2/2.12.40-2