On a Drupal 8 site, image styles are no longer generating correctly.
The sample.png used in the image styles configuration page is generated fine (including when I manually delete the file from the image styles directory). I have recursively set to 0775 and 0664 the directory and file permissions for the entire sites/default/files directory, but the error persists. The log just shows access denied.
The Drupal version is 8.1.9.
How can I fix this error?