Videos are processed by ffmpeg. When ffmpeg runs it generates an output of the coding/decoding/etc it is performing. This output; since it is ran through cron, should be sent to the email assigned to the cron or it may be sent to the email for root if an email is not assigned. Looking at the output of ffmpeg may help to track down why it is failing. I will see if I can locate a similar cron output on my server so you can see an example output.