Xbox: no media player plug-ins are installed and enabled in this project

Cannot play videos on Xbox through a mediaplayer object.

LogMediaUtils: Error: Cannot play
file://…/…/…/TheVagrant/Content/Movies/Ending.mp4:
no media player plug-ins are installed
and enabled in this project

Plays fine in editor. Startup movies also play on Xbox.
Is there an xbox.services .dll that needs to be included in Xbox One > Features > In Process Servers?

Update:
I discovered that the Media Foundation Media Player plugin was not enabled. The error has gone, but the video still does not play. Any suggestions?

Update 2: debugging MfMediaPlayer lead me to discover that SetRate() fails because TComPtr /< IMFSourceReader /> SourceReader is null. As far as I can tell, SourceReader is only set in TickFetch(), which is never called.