http://us1.samba.org/samba/docs/man/Samba-HOWTO-Collection/passdb.html grep "retrieved user": the chapter explains Plaintext passwd backend but an information what should be written into smb.conf to get this backend is missing. Is it passdb backend = plaintext ? Plaintext ? plain text ? Plain Text ? Plain text ?
There is no 'plaintext' backend. See #1684