I followed this thread after having similar symptoms, but I had a different solution.
In our case, we are migrating from 3.1 to 3.5, and unoconv seemed to be configured and working perfectly, just as it had in our old environment, but the documents in the new 3.5 environment were being rendered blank in the grading application.
The unoconv path that used to be set in System Paths, was now an item in the admin/settings.php?section=managefileconverterplugins menu that needed to be enbled or disabled. Since we jumped a couple of versions, we needed to explicitly enable that plugin.