Well, looking at the source of the current libxfixes (of Ubuntu Trusty) I can find no XFIXES version 6 code. Additionally libxfixes_5.0.1-1ubuntu1.diff.gz contains this changelog update:
-- Julien Cristau <jcristau@debian.org> Tue, 14 May 2013 10:12:48 +0200
+libxfixes (1:5.0-4ubuntu6) raring; urgency=low
I cannot test a full KDE setup here but I think at least the speculation that v6 could be the reason can be dropped. Do the kwin crashes still happen with Raring or any later release of (K)Ubuntu? (For Precise a backport might be necessary.)