Split from this thread.
We have Teams enabled in our test tenant and users have E3 licenses. Initially some weeks ago everything was working fine, but now I'm are facing issues with sign-in on iPhone. Teams App v 1.0.3 iOS version 10.1.1.
Issue appeared after I signed out from the Teams App and tried to sign to another tenant, which doesn't have Teams enabled. Thereafter I couldn't sign-in back to the test tenant, which has Team Enabled. Sign-in with same credentials via desktop client and via web app is working fine. Re-installing Teams App on the phone didn't help.