Some Mac OS Chrome Browser with hardware decoder which is not working properly may cause this problem.
Please, update Chrome setting as directions below for better conference .
1. please enter following URL in the addres bar.
[chrome://flags/#disable-accelerated-video-decode]
2. click "Disable" for "Hardware-accelerated video decode" option.
3. Restart Chrome browser by clicking "RELAUNCH NOW" button on the bottom of the page.