by AL Rachels.
After another frustrating day, I am still having the same problem. I thought sure that since you have it working with Moodle 3.1 on Centos 6.7 and MySql, I would have no problems getting it to work. Except for not mentioning php, I am using 5.5.35, your description of your setup describes my main server setup exactly.
Today I took the time to update another server using php 7 and Centos 7 and I get the same results with it. Everything appears to work, it's just the annotation window always shows me a blank white page after "Generating the PDF." I never see the contents of what the student submitted except over to the right in the Submission area, and yes, I can mark inline there. The odd thing to me is that I can go ahead and mark in the annotation window, click Save changes, and the students will see the results. If I go to regrade it, I get the page as it was when I clicked Save changes.
On both servers, unoconv works fine from a terminal window.
What's really frustrating, I just logged in as a student and teacher at qa.moodle.net and I get the same frustrating results there! I have a few more minutes before the next hourly reset before I can go check again. Going back to make sure as there were 9002 students! I guess someone may have been making a test run.