We have created a couple of custom Apps for Teams, nothing too complicated they just open Sharepoint sites. The Stream videos work fine on the Sharepoint sites themselves, but when they are viewed within the Teams App it says you are not logged in and it asks you to sign in. When you click sign in it opens a new browser tab and tries to open https://web.microsoftstream.com/authredirect nothing happens. I am certain this use to work fine, has anything changed?
We're having the same problem. Was having it in Edge and Firefox too, but not Chrome. Followed these instructions https://docs.microsoft.com/en-us/stream/troubleshooting-sign-in for edge, and deactivated the shield on the stream site on firefox, which allowed me to add a video to a office 365 power point presentation and play it back, but every time I share that presentation in a teams meeting, the video is not visible and the same sign in problem occurs. I don't know what web API Teams uses to stream MSStream or powerpoint content, but it just does not work.