import-external-images 500 error with Ajax2016Sep18
Solution was to use a simple POST – accomplished by doing this in Chrome DevTools (or similar JS console) when logged in:
jQuery.ajax('/blog/wp-admin/upload.php?page=external_image', {method: 'POST', data: {action: 'backcatalog'}};
The plugin needs to be fixed. I swear it used to work.