I've been searching around for days to find the clue for the NT_STATUS_LOGON_FAILURE, I found that it is wrong user/password. But I've discover that if you don't have a hostname set for the server this error appears. Comment the line hostname="XXX" form the /etc/rc.conf and no login is possible except for anonymous connection. You should notice at startup (/usr/local/etc/samba.sh) that no hostname has been set and it will cause problems.
Actually, if you explicitly set the 'netbios name' option, this should not matter. I think maybe this is something for the FReeBSD package/ports maintainer. As a general rule, Samba has to assume a correctly configured OS.