This app is not currently accessible and the app developer is aware of the issue. You will be able to log in when the app is reactivated.

I have told you about the restriction. Please double check your app is restricted by Facebook or not. This problem is not related with facebook app approval.

It looks like your Facebook app is restricted by the Facebook team. So please apply to get back your app approved to the Facebook team.Or follow this blog and create the app again https://xeroneit.net/home/blog/xerochat-how-to-submit-app-for-review