Video Webm video - transcoding not working

Hello all,
I have to use a video with transparency channel in my project.
According to this article https://docs.unity3d.com/Manual/VideoTransparency.html
I've created a Webm with VP8 video, and it work fine in Editor.

But I should use it on Android platform, so always according to the previous article I've checked "Transcode" in "override for Android", on Inspector settings.

The problem is that after transcoding I get the Console error message
"WindowsVideoMedia error 0xc00d36b4 while reading C:/Workspace/Project_a/UnityProject/Temp/importedVideoClip.m4v"
"VideoClip import error for Assets/Scenes/Movies/output.webm"

and the video is not readable anymore in Editor (and, so, it's removed from VideoPlayer).

What I'm missing?
Many thanks!

 

I've found my mistake: I left "Auto" in transcoding codec.
Now it work!