It sounds like the file may have become corrupted. Does this happen to all videos, or just a certain one?
Do you remember if the game session that it was recorded from was able to exit correctly? For example, it didn't end with a game being forced to shutdown quickly, or a PC crash.
It would be rare for it to affect that many sessions if it was a crash. However, it's still something we can look into.
I would recommend creating a support ticket if you have not already. If possible, feel free to mention that we spoke through the forums.