I have same problem, just got this samsung 8 series tu850D for Christmas, it worked from mid Dec to yesterday Jan 23 2021. Called Samsung support, they deleted & reinstalled xfinitystreaming app. Worked one time until I closed & tried to reopen it. Spent over an hour with comcast tech support but they tried to tell me my tv wasn't supported by the app. There is an xfinity feature that confirmed my new tv is supported. I called comcast to disconnect in Nov 2020 because the boxes you rent are too expensive. The agent told me to buy a new smart tv, get the app & return the box. I did that & spent $599 at Costco. Now I have returned the box & can't watch my new tv. NOT HAPPY 😞 thanks alot Comcast/Xfinity or whatever you want to call yourself.
Same thing for the last couple of weeks...
Symptoms: After switching to another Samsung TV app and returning back to the Xfinity Streaming App, a blank black scrreen and white/gray line at the top. Temp fixes found: