Black screen when playing flash video on SOME sites

This is using a MBP 2011 with OSX El Capitan.

I have flash 19.0.0.226 installed (NPAPI and PPAPI) and there are many sites, such as CNN.com, where the videos are just a black square.

When I load a page with a video on CNN.com (for example), here is what happens:

- The first frame of the video appears

- The first frame is replaced by a black area (but the right most column) as if an overlay was put on top

- Then the right column disappears

- When I press the controls, nothing happens; no video, no sound, no network IO: the video does NOT stream.

I have tried:

- Firefox and Safari

- Firefox and Safari in 32 bits

- Creating a new Firefox profile

- Running Firefox in safe mode, without any plugins

- Disabling hardware acceleration

- Running the flash remover tool and reinstalling

- Clearing the flash cache

- Forcing the use of the AMD HD6750M GPU

- Forcing the use of the Intel HD Graphics 3000 GPU

Flash will work properly on many sites, will play videos properly, etc

But on some sites it is just that black rectangle...

Same site on the same computer, using a Windows VM will have the video play properly, so this is not due to my location, etc

Same site on a different Mac will also play properly, from the same IP.

What should I try next?