Had suggested changing ownerships/permissions more liberal to trouble shoot.
select * from `mdl_files` where `filename` like "cross.png"
You'll see several references - 4 per assignment (one for each of the icons) me thinks.
So your server is configured to use unoconvert with libre office installed and the listener configured and running properly?
One of the things I noticed when working with that ... if there were multiple documents submitted for the assignment the generation of the PDF for viewing in the grading interface was confusing ... in the panel for the display of the PDF (single) generated hard to determine where one document ended and the next began.
Lot's of folks have had issues with this and opted to use Google Document Conversions instead.
What do you see in moodledata/temp/assignmentfeedback_editpdf/stamps/
Would think that IF your moodledata directory is in a secure location more liberal ownerships/permissions might be ok ... even if it means the app can do things .... but that's my 2 cents, of course.
'spirit of sharing', Ken