I needed to include the "b." prefix in the Project ID, regardless that it was an ACC project.
https://developer.api.autodesk.com/data/v1/projects/b.4e97ffae-b501-4ebd-8747-98206589e716/folders/urn:adsk.wipprod:fs.folder:co.szzRe5O9Q12iXBKOtKlmZA/contents