FB app created successfully, but getting "enter correct Facebook API settings" message in

Hello,

When entering the API info in Ultimate Facebook settings, I get the message "Please, enter correct Facebook API settings"

As far as I know, I've successfully created a FB app following the exact steps in the guide. The keys are entered correctly.

I'm using the plugin Absolute Privacy to lockdown the entire site and require a login. I disabled it but still had this issue. Any idea what the cause might be?

Any help would be grand. Thanks!