An Error 530 occurs because the username, password, or account group on the Endpoint does not match the information for the Endpoint on the server.
In the RBS Server, you can check this user's details in the Edit Endpoint page. On the Endpoint, you can check these details by going to Options -> Preferences and then double-clicking the padlock icon and entering the password (Admin with a capital A by default). To check the Account Group, click on Backup Server URL to change this field to Account Group.
If these three credentials (username, password and account group) match, check for any spaces in the fields and try retyping the password. The username/password fields are also case sensitive, so make sure these are typed in with the same case as what appears on the RBS Server.
If everything is correct here and you still get a 530 error, it may be due to permissions with the directory on the RBS Server where you are storing this Endpoint's files, OR with the drive itself. Make sure the drive is attached and mounted. Check the RBS Server for the Root Director for the endpoint. You can find this in the RBS Manager Console Application.
Check the permissions on the path and make sure that the local SYSTEM account has full control if you are running as a service or that the logged on user has full control if you are not running the RBS Server as a service.
A bad data drive, or a drive that has become dismounted will cause this error.