Getting a grey screen in our app, not getting resolved even after debugging

How do you expect us to help you with that?

What are the users logging in to? Is it your own backend system? If so, then it's up to you to figure out whether the service is returning the right value and that your app is receiving it and handling it properly.

If it's some other system, then which system?

You've literally just mentioned that there's a bug somewhere and you expect us to be able to solve it with the vague details you've provided. I think you may need to provide way more details than that.