It's not the file itself because I've played it on the computer and rockbox before.
Have you played this exact copy of the movie before on Rockbox? On version 3.7?
I'm just trying to narrow down what changed.
(As far as I can figure) Either:
1 - you are mistaken when you say you've played this file on Rockbox before. (please double check the codec used)
2 - There is filesystem corruption on your player and the video is damaged.
3 - There is a bug in Rockbox in 3.7 which was not present in whatever version of Rockbox you played the video on before.
Unfortunately, the ball is totally in your court to investigate #1 and #2, and we need more information from you to investigate #3.