If you’re using WWW for ftp, its possible it may not work. 530 is a authentication error, you can try to include your username and password in the URI:
and see if that helps. Storing your username and password… and using it in a url will expose it to reflection and others may obtain your credentials for the ftp server. It would be better to setup a service, whether that is a restful like service or just some PHP script that responds to calls to serve up your assetbundles.