Just so people in the WordPress.org forums can see your reply:
“Double check that the red URL the plugin indicates on the settings page is exactly the same as the Facebook Website > Site URL and that the Facebook App Domain field is empty. Note that you have to click on ‘Website’ to ‘open’ this field.”
The second line of my first post stated that the URL is identical, having been directly copied and pasted from the ALTF Settings page.
The type or brand of hosting does not matter. The URL Facebook is requested to redirect to by the plugin should be exactly the same as the one configured on Facebook. The plugin requests a redirect to the red URL shown and therefore this should be exactly the same on Facebook. This is an important Facebook security measure.