The course was created with the "rerun this course" feature.
For units with unpublished content, "Preview" always takes the viewer to the page "PAGE NOT FOUND The page that you were looking for was not found. Go back to the homepage or let us know about any pages that may have been moved at firstname.lastname@example.org."
To me, it looks like the "preview" button is trying to take you to a link that looks like how the original course constructed link locations (i.e. .../courses/[course]/[number]....), not like how the new rerun course constructs link locations (/courses/course_v1:[course][number][run]...)
1) In a rerun course, edit a component
2) Click preview
1. As of writing this ticket, here are units with unpublished content, as well as the link you get when clicking "preview"
Try preview in:
Get (page not found):