I am no longer experiencing the issue.
I have my blog hosted on Pantheon, with a reverse proxy through Fastly.
When trying to access the Bulk Optimization plugin via our own domain name, images would “cancel” and I would get console errors
503 error: /wp-admin/admin-ajax.php
and
Cannot read property 'image_sizes_optimized' of null
But when I access the plugin via the admin panel directly on the Pantheon environment, and not through our domain, it works no problems.
