I've made a number of changes (upgraded LibreOffice, unoconv and gs as well as changed the home dir for the apache user so that it is writeable). And the problem still persists for the .docx files that I'm trying to convert to PDFs. However, its not persistent across *all* .docx files; some convert fine. I think the problem might be that the original, corrupt conversion is being cached somewhere and that the unoconv plugin in Moodle determines that the converted file is already present and does not try to regenerate it.
What I'd like to know is, does Moodle cache the converted PDF document and, if so, how can I find it?