I'm not sure but some include/exclude dialogs in Corona also include the hierarchy linked to a mesh. If your mesh is part of any hierarchy (is a parent or child) try to unlink it first and see if that helps. Might also be that the Railclone object that uses the path creates the dependency loop, in that case you'd need to copy it and use the copy as the distance object.
Also, if the path object uses the material the distance map is used in, that will throw an error too.