Same problem here.
I am playing a Web file and the game crashes about 0.2 seconds after I start play, but no video ever shows.
The problem only occurs with VP9 encoded webm files on Windows. A VP8 encoded file from webmfiles.org worked just fine.
The exact same code works on Linux and on the Oculus Quest so it seems the VP9 decoder on Windows seems to be the problem but it does not give a debug output and no error message it just crashed silently.
Here is a sample project to test it: https://drive.google.com/file/d/1ak4PgYoql7YPQO2nVQkoBd94obibTGHW/view?usp=sharing
It starts with the video that crashes on Windows if you want to verify that the code can play a working video change the line
in Main.gd to
So to recap:
Windows does play VP8 webm but crashes VP9 webm
Linux does play VP8 and VP9 webm
Oculus Quest/Android does play VP9 webm (VP8 not tested)