Can't you set the Loglevel in httpd.conf to debug? (I'm using FreeBSD)
ee usr/local/etc/apache22/httpd.conf
change loglevel :
'LogLevel: Control the number of messages logged to the error_log. Possible values include: debug, info, notice, warn, error, crit, alert, emerg.'
Try changing to debug and re-checking the error log after that.