Unable to load the registration form

When a user who is not currently logged into Facebook goes onto my Registration page they are currently confronted with the following error message:

Unable to load the registration form for Thrive. You may have previously blocked this app on Facebook. Go to your Facebook privacy settings to unblock this app. (Error: Invalid 'client_id'.)

If the user is currently logged into Facebook (in their browser) then the form appears as it should.

I have the plugin set so that users can only register with their Facebook account and the default Wordpress registration page is being overidden by the Facebook one. What do you think the problem is?

Despite reseting my API Key the UF Plugin is also giving me the error message:

Please, enter correct Facebook API settings

I am sure that I am inputting the correct details. What do you think the issue is?