I had a few .mov video files in channel 1, and it rendered fine, but then I tried to add an mp4, and I got an error (xyz.mp4 could not be loaded), so I decided to add some .jpg images instead.
The images rendered fine, but when they weren't on screen, it would render as black. Even when I removed the images, it would render as black.
I tried duplicating one of the strips from the first channel and putting it into the second channel (hoping is was just an issue with the channels), and it still had that issue.
But when I added a new .mov, it would render that one fine, but it wouldn't fix the other strips.