If you create a "directory" via the AWS web interface (as you might do before populating the directory with objects), a one-byte object results. It can be seen along with contained objects with:
If this object is present, it prevents the directory from being listed, seemingly because Lucee finds that "directory" object and thinks it's a file (which would explain the error message).
One way to get the directory Lucee-listable is to delete the one-byte directory object with:
...and the "directory's" contents are preserved.
I haven't found a way to delete that object from Lucee, though (neither fileDelete() nor directoryDelete() seem to have any effect).