I just tried this out, and with secure media set up and everything my large image upload is working fine, and 5 optimized images are created with the correct security. Looking at how these are generated, I am wondering if your responsive_post_image_sizes or create_thumbnails site setting has been changed?
Do you get the Lightbox at all when you upload an image that is too large and should be optimized? Note that if an image is too small it will not get optimized images generated for it.