Apologies for the late response and also for the frustration & difficulties you faced with the new Adobe Express. Do you still see the issue of the video preview not being played? Are you getting any error messages?
Have you tried using the other suggested browsers ( Microsoft Edge, Google Chrome) to see if it makes a difference? Try clearing the browser's cache and cookies to check if that helps.
Also, try restarting your computer.
From Mac browser: Cmd + Shift + Delete
Hard refresh: Cmd + F5 or Cmd + R.
Please let us know if this helps. It also helps us with a screen recording of the workflow to investigate further.