So looks like Milk Factory royally screwed up on the preview video for Oppai Isekai on their site. It can only play the low quality version on their site and the 60 FPS mp4 links are all invalid.

See for yourself: https://milkfactory.jp/products/hmaho/movie.html

Luckily I saved the 60 FPS version of the preview video 2 years ago. This file is known as "e_maho_pv_60.mp4"