Aperture some video thumbnails are black

Videos in Aperture vers. 3.4

After sometime some, but not all, video thumbnails turned black. The videos are still there and display correct when viewed.

The videos with black thumbnails are generally from iPhone.

Question: How can I get the thumbnails back again.

I have tried the following, without any success:

1. Manually set the key frame (right click on video)
2. Repaired permission
3. Repaired Database
4. Rebuild Database