I created a new collection, Pathfind, which lives at https://observablehq.com/@tmcw/-/collection/pathfind - and currently it’s empty. When I click “Add Notebooks” on that collection page, I arrive at the URL https://old.observablehq.com/collection/@tmcw/pathfind on old.observable, which is a 404. Looks like a bug in the migration from old to new.
Hi Tom. Is it possible that you’re just signed-out of Old Observable? You have to sign-in to Old Observable separately, and since your collection is private, you’ll see a 404 page if you’re not signed-in.