Duo MFA prompt not displayed on MacOS or iOS

After providing username and password, the Duo MFA login prompt doesn't appear. Instead, an empty web page appears with https://admin-906cc653.duosecurity.com/ in the URL field.

This error is often due to content restrictions, or other security measures, enforced on Apple devices. To resolve this error, check the following: 

1. Clear browser cache & cookies, and use private/incognito mode - instructions
2. Disable content filtering/blocking - instructions
3. Be sure to accept cookies from duosecurity.com - instructions
4. Enable Javascript in your browser - instructions
5. Turn off Apple's "Private Relay" feature - instructions