Version of Moodle? ... as well as typical tech info ... PHP version, running what as web service, etc, etc. How many servers do you have in "GauchoSpace"?
Have you checked server error logs at times when doc conversion fails?
Prepare submissions for annotation \assignfeedback_editpdf\task\convert_submissions
Set to run /15 by default - yours?
Have 'run now' link there?
Is cron job and adhoc task running as expected?