The preview image of the video is way too large (10000x10000 px).
NOTE: i fixed this on this live website by adding this css:

#portfolio-extra img {max-width: 100%}

This is Salient version 15.0.8, downgrading to 15.0.4 fixes the problem! (without my extra css ofcourse)

I first thought it was cache or css compression, but it turn out that changing the Theme version back fixes the problem, so there must be something different in v15.0.8. I could not find any differences in CSS for the video or parent html codes.