It is a common apache problem.  Elementree imports an expat parser
(presumably cElementree doesn't) and different versions of expat play
together very poorly.  Lots of apache modules load one xml lib version
or another and they tend to step on each others toes .. and then segfault.

IIRC the only way to resolve this is to figure out which loaded apache
modules are using expat and recompile them all against the same version.
 Not fun.