It seems like a lot of people has been having issues with this, and unfortunately, I've also been a victim. However, the "fix" was changing / reverting settings for .mp4 extensions under 1080p. I don't use that. As to reverting the settings, all my other videos are rendering fine with my render settings, and I've no wish to change it back. I've checked my hard-drive, it still have 300 gigabytes left, and the total render of the video (~27 minutes) is only 4 gigabytes. Any ideas? I would have continued to search for fixes, but I have to go soon, so again, sorry to bother all of you. Thanks for any responses.