Samba share authentication

[blackjackchik]

Hello. I have Samba server (Ubuntu 7.04) an two clients (Ubuntu 7.04). Samba must authenticate group users when they make connection to samba share. It`s work fine but when i try connect and authenticate to share simultaneously first client saccesfuly login and second client login fail.
It`s my config samba

[global]
name resolve order = wins bcast hosts lmhosts
passwd chat = *Enter\snew\sUNIX\spassword:* %n\n *Retype\snew\sUNIX\spassword:* %n\n
add group script = /usr/sbin/groupadd %g
delete group script = /usr/sbin/groupdel %g
add user to group script = /usr/sbin/usermod -G %g %u
logon drive = H:
username map = /etc/samba/smbusers
encrypt passwords = yes
passwd program = /usr/bin/passwd %u
passdb backend = tdbsam
wins support = true
dns proxy = yes
netbios name = samba24
server string = %h server (Samba24)
logon script = scripts/logon.bat
default = labs
local master = yes
workgroup = labs-ef.local
logon path = \\samba24\profile\%U
os level = 255
add user script = /usr/sbin/useradd -m %u
security = user
add machine script = /usr/sbin/useradd -s /bin/false/ -d /var/lib/nobody %u
delete user script = /usr/sbin/userdel -r %u
log level = 3
domain logons = yes
[labs]
locking = no
valid users = @users
writeable = yes
path = /home/shares/labs