There is a chance that the video you are using is not supported by our encoder.
First, go to Upload Settings and once the Upload Settings open, click on Edit Videos.
If "Edit videos before uploading" is checked, then try to ADD A WATERMARK.
If you already tried to add a watermark, then uncheck this option (there is a chance that adding watermark fails).
If you are trying to cut time from a video, try to uncheck the option and see if the video uploads.
if you are trying to append an intro video, make sure that the intro video has a background audio on it (make sure the intro is not silent). To eliminate this problem, try to NOT add an intro video and see if the video uploads.
Take each option by exclusion and see what is causing the problem.