I have Skype for Business application set up and ready to use. I can sign in with Administrator user and I already gave Admin Consent to the app.
But, with other users(not Admin) I can't sign in. What may be the problem here?
So, the problem was (a bit silly I must admit) I didn't have any SfB subscription for the other users. That was why I couldn't sign in except with admin user.
After we purchased more subscriptions, there were no problem.