There are plenty of bug reports open on this, but it's pretty clear that the KDE/Kubuntu guys either have no clue how to fix this, or cannot be bothered.
Since knotify4 isn't really all that useful (especially when using E17 for a WM), there is a brutal hack that will effectively silence it:
sudo mv /usr/bin/knotify4 /usr/bin/knotify4.orig
sudo cp /bin/true /usr/bin/knotify4
Needless to say, the original file should be restored before doing an upgrade.