We have a storymap with multiple videos, with playback controls enabled. For one of the videos, if viewed in fullscreen mode (using the button shown below), the screen goes black. The other videos in the storymap appear unaffected.
I can however successfully view the video in fullscreen by right-clicking and selecting Open video in new tab. I am experiencing the same behaviour in Google Chrome and Microsoft Edge browsers.
The affected video is a screen recording captured on my phone (Samsung running android), and then edited using the Windows Video Editor app. It is mp4 format.
I apologize that you're seeing a problem with one of your videos. I'm not able to reproduce this, so it doesn't appear to be an issue with all videos.
Please contact Esri Technical Support and they can further assist you in troubleshooting this issue.