I fall in this bug when I had tried to change default password of WD_Backup user by Web UI.
Web UI goes in error using password different then the default one (“backup”), it messes up in the /etc/passwd file with the WD_Backup entry so the user is not to able login anywhere, anymore.
I found a fix in this blog
http://dreamrivers.wordpress.com/2013/01/05/mbwe-repair-wdbackup-en/