by Nathan Rosenquist.
Searching around the forums and tracker it seems this question has been asked several times and gone unanswered.
If unoconv is installed on a moodle host, moodle can be configured to use it to convert uploaded assignment files to PDF for web-based annotation and grading. If unoconv fails to convert the file for any reason (Like the unoconv listener crashes), a blank document is generated instead. Once unoconv is fixed and document conversion resumes successfully, these blank documents persist and thus unoconv doesn't re-attempt to convert the uploaded file upon revisiting the assignment page. This is the issue I'm currently having in Moodle 3.1.1.
I have attempted purging caches multiple times. I've also visited moodledata/temp/assignfeedback_editpdf and moodledata/temp/core_file/conversions, and removed all .doc / .docx / .pdf files from there. Apparently none of these methods clear the cache location that unoconv is storing these converted assignment files in.
Does anyone know what I should be doing to flush out these old failed conversions? PS, I've already taken the student uploads that failed and submitted them to a hidden course/assignment just to make sure that they do indeed convert correctly now, given the chance. I doubt the assignment itself is the issue here.