For some time, the effect »Translucency« was listed twice in the KWin KCM plugin list of my local KDE setup (SVN trunk). One copy was actually working, the other was just producing error messages.
Today, I finally had time to investigate the issue: The problem was caused by a stale
.desktop file in
share/kde4/services/kwin with the old name of the plugin (it was renamed from maketransparent to translucency).
I have no idea how this could happen, because I usually purge the whole
/opt/kde folder everytime I
svn up my
qt-copy, which I happen to do quite often…