Error message attempting to stream Apple TV rented videos to TV

After successfully streaming a number of movies rented through Apple TV to my Samsung Q80 TV, it suddenly stopped working. Using our MacBook Air, screen mirroring works until I start to play the movie, then both the computer screen & the TV go blank, although the sound switches to the TV. When I attempt to do the same thing using movies/TV programs that I have purchased & previously played successfully to the TV using my iMac, I get the error message "This video is in a format that can't be streamed to this TV". Videos streamed thru Britbox are ok.

I have recently upgraded to High Sierra on both my iMac & MacBook Air, but cannot remember if I had any successful movie viewings after doing so. The Samsung Q80 TV is new & has Airplay.

What is going on???