As far as I can see.... if an incorrect unoconv setup (e.g. incorrect permissions) causes a failure of the test asking to download the test PDF results in a 'must be fixed by programmer' error. However, there's no way to 'reset' this. You will now get this error even if you fix the original problem.

Resetting everything with...

<?php require('config.php');

 $fs = get_file_storage(); 

 $fs->delete_area_files(context_system::instance()->id, 'core', 'documentconversion');

...allows it to work.  This makes it very difficult to set up unoconv unless you are lucky and get it all right first go.