If you're using wfDebugLog (and not just wfDebug)
as per
http://www.mediawiki.org/wiki/How_to_debug#Logging, then I suggest you
try changing the log file location to somewhere below htdocs as in the
example on that page - my experience in this area is limited, but I'm
not sure whether an arbitrary location is OK.
The log location is definable by the end-user. I've used /tmp before, and it
worked perfectly fine. It really depends on the web server configuration
though.
V/r,
Ryan Lane