I have installed and setup PDFPreview module according to the instructions. The PDF attachment file name has disappeared from the article pages with nothing in its place. When I examined the HTML, there's an img tag pointing to the preview image, but it doesn't work.
When I access the preview image directly, it produces a 500 error with a (translated) error message indicating an error while producing the image. The Drupal log at Reports > Recent log messages contains the error message
Unable to generate the derived image located at public://styles/medium/public/pdfpreview/3b350fd949abe27c0778b0f857ffa352.jpg.
I found this error report and tried the steps instructed in message #2. I created the directory sites/default/files/styles/medium/public/pdfpreview/, have tested directory permissions up to 777, and tried switching the 'Image style' preference back and forth with no success. The web server has ImageMagick 6.7.7-10 installed.
I have no idea how to even debug the issue further. The Apache error logs contain nothing, and the Drupal error message doesn't provide any indication on what has caused the failure.
How can I further debug the issue?
Detailed Drupal error log:
(I have since reverted the configuration, so that users at least have the file name link visible.)