Back from vacation. I peeked at the source code:

            if (relative) {
                if (!scope.containsKey("imageNum")) {
                    throw new ElementException("Relative indexing in ja:fileiterator not allowed in this scope", element.getContent());
                }
                int current = (Integer)scope.get("imageNum") - 1;
                start += current;
            }

In plain English, what produces this error is if you try a relative iteration without the presence of the "imageNum" variable. That can, for instance happen if you issue a relative iteration in a custom page or in index.htt, outside of a ja:fileiterator element.