Need help with Handeld.PlayFullScreenMovie for Android

okay, there were like dozens of threats about this topic and I've read all of them at least twice. But I still can't get it to work. So here's my Setup:

1) In my Assets folder I've created another folder called StreamingAssets. 2) In this StreamingAssets-folder there's a H264-MP4 file called animation.mp4 3) In my scene I've got a c#-script attached to an object:

 void Update () {
     if (Input.GetMouseButtonDown(0)) {
             playMovie();
     }
 }
  
 void playMovie() {  
     Debug.Log ("Click registered");
     Handheld.PlayFullScreenMovie ("animation.mp4", Color.black, FullScreenMovieControlMode.Hidden);
     }
 }

When I test the app on an Android-Tablet via Unity Remote 5 I can see the background color of the camera (that's fine). When I tap on the screen, my Console says "Click registered", but the video isn't playing, I can't see anything happening at all.

Do you've got any ideas what I am missing/doing wrong?

Kind regards, Paddy