"Too many inaccessible URLs from the same domain", what is this?
If some of the images were optimized and at one point ShortPixel stops optimizing and then the error "Too many inaccessible URLs from the same domain" appears, then please refer to Error: "There was an error and your request was not processed".
If that doesn't help, and the error is happening on all your images, make sure to read this article: None of my images were optimized, why is that?