Usually, that means that there is an error with the website (i.e., YouTube or Vimeo) that is hosting your video. In that case, the battle video can be replaced, if requested. Just let us know.
Simply hit the "Report" button underneath the battle video -- or submit a support ticket directly at
http://www.theory11.com/support -- to report the technical issue. If the video needs to be replaced, be sure to provide an alternative link for an identical, working battle video. Battle videos, of course, cannot be replaced once a battle has begun for any other reason than technical.
Hope this helps,