Answer
Double-check that the Oracle DIRECTORY objects' path values exactly match how the share is actually mounted on the database server's own filesystem (not the path as seen on the file-share server itself) — a DIRECTORY object pointing at a path that doesn't correspond to a real, correctly-mounted location on the database host will fail file operations even though everything looks correctly configured from the file-share side; recreating the DIRECTORY objects against the confirmed, correctly-mounted local paths resolves this class of failure.