samba - Jul 1999 - NT4 SP3 and Samba - Connection problems

Hello All,

First, I am new to Linux.

I am having troubles with NT4 SP3 and samba-1.9.18p5: some NT4 nodes can
map Linux and/or Unix drives, some others can't.

The errors messages are:
- this account is not authorized to login from this station
or
- incorrect password or unknown username
(but I am sure that login/password are ok)

I have already check the EnablePlainTextPassword = 1 in the NT
registries.
I tried also to use the debug options for smbd (using smbd -D -d 5 -l
/tmp/smbd.log) and nmbd, the only interresting thing in the log is:
...
switch message SMBtconX (pid 649)
Got device type ?????
Scanning username map /etc/smb.usermap
Trying username Map-Name
Scanning username map /etc/smb.usermap
SMB Password - pwlen = 8, challenge_done = 0
Checking password for user nobody (l=8)
1999/07/28 10:06:05 invalid username/password for Map-Name
1999/07/28 10:06:05 error packet at line 171 cmd=117 (SMBtconX) eclass=2
ecode=2
error string = No such file or directory
...
switch message SMBtconX (pid 645)
Got device type A:
Scanning username map /etc/smb.usermap
Trying username Other-Map-Name
Scanning username map /etc/smb.usermap
SMB Password - pwlen = 9, challenge_done = 0
Checking password for user UserID-Name (l=9)
ACCEPTED: session list username and given password ok
found free connection number 118
Scanning username map /etc/smb.usermap
...

In the first example the login fails and in the second it succeeds.
Any idea ?

Here is my /etc/smb.conf 
[global]
        max xmit = 8192
        log file = /var/log/samba-log
        max log size = 5000
        username map = /etc/smb.usermap
        workgroup = WorkGroupName
        security = share
        public = no
        writable = yes
        users = @GroupName

[Map-Name]
        comment = Map-Description
        path = /Map-Name
        public = no
        writable = yes
        users = @GroupName
....


Many thanks in advance,
Ciao,
Jean Philippe