Being asked for username and password for NAS shareer

I’m trying to set Krypton 17.1 up on my Nvidia Shield TV box, but when I try adding the shres from my NAS for movies and tv shows, I’m being asked for username and password details. The folders are in the Public share, which is not password protected. And when I try to use my own user account, or Administrator, details I just keep getting returned to the username and password box again.

Can someone give me some advice on what I need to do?