Double clicking on MP4 displays black screen not movie

[Mshl0357]

I am using Lightroom Classic 10.1.1 on a MacBook Pro running MacOS Mojave 10.14. Today when I double-clicked on any MP4 file in my Library, the Loupe view opens but the video  display area is black. When I click the "play" button the progress indicator moves but the screen is still black. If I click the Trim button the video frames show and when I click play the progress indicator moves across the frames, but still the large area above the Trim display is black.

I know I have played videos before in LC but I don't play a lot of them so I don't know when this problem began. 
Any advice is greatly appreciated!!

[johnrellis]

Have you restarted your computer?

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[Mshl0357]

Yes. I have shut it down several times over the last few days but problem still exists.

[johnrellis]

Video has been LR's stepchild since it was introduced in LR 3, with continual reports of playback issues. Maybe others will have better suggestions, but if you look at the many other threads about video in the forums, lots of unsolved problems.

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[slloyd47]

Did you ever figure out the solution to this problem. I am experiencing the exact same thing. Any info appreciated.