The issue has recurrred: endless signin required. The problem resurfaced May 29, 2019 around 3:00 PM US Central Daylight Savings time. The PBI PowerApp opens properly in the Teams Web browser but not in our local corporate Teams.
As mentioned in my earlier post, I have a Power BI report with an embedded PowerApps visual. It works in the Teams web browser.
In the Teams desktop app it gives the sign in message "Sign in to start using PowerApps." When you press the sign in button, no sign in window opens.
The same thing happens in the Teams mobile app.
The problem was fixed, it resurfaced on May 29th.