See http://us1.samba.org/samba/docs/man/Samba-HOWTO-Collection/passdb.html grep "retrieved user": it speaks about Plaintext passdb backend. However man smb.conf, passdb backend entry 1) Doesn't list plaintext passdb backend in the available backends 2) Doesn't have plaintext passdb backend in the examples
There is no 'plaintext' backend. See #1684