What happens if you change your "not-existent" password to be the same as used on the NAS for the same user?
Just for a test..
But if you don't want take the risk of loosing your password/data to thieves, any automatic connection to the NAS with locally stored credentials should be avoided. What about creating a dedicated user on the NAS and whenever you launch the backup tool, connect to that share or what it is by manually creating the connection first?
Or connect to the sensitive locations manually at the time your computer starts up, that's what I do, to get around storing/reusing existing account/password information on the system.