While I'm hitting this issue (again :-) ), so I want to document this here...
My problem was that I had a blank smbpasswd. Even when I added the passwd manually with 'smbpasswd'. After a reboot, the password settings in /var/etc/private/smbpasswd are gone.
With debugging /etc/rc.d/smbpasswd (with set -x) I've found a variable called 'smbpasswd_minuid=1001'. My UID's are bellow 1001 (I'm using the same as on my Mac).
So after some search, I've seen that this feature (Enable customizing of minimum UID for smbpasswd via rc.conf variable 'smbpasswd_minuid'.) was implemented with Build 3177.
It is easy to set this variable in -> System -> Advanced -> rc.conf
Variable = smbpasswd_minuid ; Value = 500
After applying this, it is possible to run /etc/rc.d/smbpasswd. The users and the encrypted passwords are stored in /var/etc/private/smbpasswd.