The following solution article is for Apple Devices and it's for users who signs into ATU Single Sign-on but Duo Mobile window only shows a white blank box.
Solution 1: If your experiencing this issue on your Apple iPhone follows these steps. (iOS 12 or newer)
Step One: Go to the setting > Screen Time > Content & Privacy Restrictions > Content Restrictions > Web Content.
Step Two: Uncheck Limit Adult Websites to Unrestricted then back to Limit Adult Websites.
Step Three: Go back to your phone browser and refresh the page. The Duo Screen will pop up for you to authenticate.
Solution 2: If your experiencing this issue on your Apple MacBook follow these steps. (macOS 10.15 Catalina or higher)
Step One: Open System Preferences
Step Two: Go to Screen Time > Preferences > Content and Privacy.
Step Three: Set web content restriction to Unrestricted Access.
Click here for a direct link to the Duo help solution article.