Further investigations have found that long events don't work well in the new Pro Player on the Xbox - the Xbox takes too long to start the video, by which time its permission to play has expired.
With Fight Night being over 4 hours 30 minutes long, most users were experiencing this issue from the Xbox.
Other devices are working as normal, with video starting in just a few seconds.
We've temporarily disabled the Pro Player on Xbox whilst we work with Microsoft to find a way to make the video start playing faster. This means you'll only be able to watch events live, or on replay (after they have finished) - you'll not currently have the option to rewind or start it from the beginning until after it has finished.
Sorry for the inconvenience.