There is an error, where I'm wondering if that is really a shapefile issue or one from our site. I'm wondering because of the error message and because the shape displays just fine on mapshaper.org.
It concerns this file: https://cityloops.metabolismofcities.org/library/829097/?create_shapefile_plot=true, (also for the same one in the library) and I've also observed it for others, e.g. this one, which has in the meantime been deleted.
Paul, could you please confirm one way or the other what the problem is? If you have any idea how this can be resolved in case it is a shp issue, please let us know too. Thanks.

The way it looks on mapshaper.

At first sight it seems to be a problem with the preview generator software, not with the shapefile. I would need to dive deeper into this though. But if it shows properly in mapshaper, we can assume the shapefile is fine.

Ok, thanks a lot Paul! This already helps us for now :)