Sometimes a video stream in a video call is completely black. How can we prevent this from happening? This also happens with the Communication Services Package Sample App which is included in the SDK.

If you gather debug logs of such a call (from both clients) it might provide a clue
https://www.devconnectprogram.com/site/global/products_resources/avaya_client_sdk/programming_docs/current/javascript/communication/tech/troubleshooting_and_debugging.gsp
If using window.console as your logger be sure to enable verbose logs in the chrome devtools before capturing the logs.