When you fulfill an RBdigital audiobook you get a manifest with links to individual chapters. These links look like this:

https://circulation-manager/works/123/fulfill/45/0

The name of the library is missing from this URL. It should look like this:

https://circulation-manager/library/works/123/fulfill/45/0

Without the library name, the patron can't be authenticated, so we can't tell whether the patron is authorized to get this audiobook chapter.