Facebook app is temporarily disabled
(#200) Access to this data is temporarily disabled for non-active apps or apps that have not recently accessed this data due to changes we are making to the Facebook Platform. https://developers.facebook.com/status/issues/205942813488872/
Facebook is making changes in their APIs and during that process, they temporarily disable apps for a while. The app should be enabled in a few hours. Until then you won’t be able to log in or register with your Facebook app.
Insecure Login Blocked: You can’t get an access token or log in to this app from an insecure page. Try re-loading the page as https://
All Facebook APIs created since March, 2018 will have the Enforce HTTPS turned on. This means that Facebook will accept only HTTPS for OAuth Redirects.
To make it more simple, if your site runs http, for instance, http://example.com you won’t be able to allow the login or registration until you get SSL for your site. (So your site URL becomes https://example.com.)
You can learn more about the topic at the official Facebook docs.
How to add SSL to WordPress?
You can find great explanations and tutorials on the topic on the following sites:
Strict URI matching
In March, 2018 Facebook will turn on strict URI matching which means that all redirect URI’s used by an app need to be listed in the Valid OAuth redirect URIs list in the app’s Login Settings.
You should check your Facebook app and make sure that the correct Valid OAuth redirect URIs is put into the field. You can get your Valid OAuth redirect URI from the Facebook provider > Getting started page, at step 11.