The documentation https://backendless.com/documentation/files/rest/file_renaming_a_file_directory.htm page should be updated:
Current: current-name- a path identifying file or directory to be renamed. The must start with the root directory for the file storage allocated to the application.
New: current-name- a path identifying the file or directory to be renamed. This must start with the root directory for the file storage allocated to the application.