Error "URL is inaccessible from our server(s) due to access restrictions"

When you optimize an image with ShortPixel Image Optimizer, you may receive the message "URL is inaccessible from our server(s) due to access restrictions".

This error message appears when our servers cannot download the image from your website. The most common causes are a website protected with HTTP AUTH, a firewall, or other server-side blocking. For all possible causes and troubleshooting methods, please read this article: Why some or all of the images cannot be optimized?