[Samba] PDC over PPTP VPN

[shacky]

Hi.

I'm trying to add a client running Windows XP Professional SP2 to a PDC
server running Samba 3.0.13 over a VPN tunnel made with PPTP.
The VPN tunnel works without problem, and I can see the shared resources on
the PDC from the client searching the server with its hostname (I can access
to \\server).
When I try to add the client to the PDC, Windows XP ask me for the root
user, but after some seconds I get an "invalid username or password" error.
The strange thing is that the username and password are correct (I checked
them a lot of times) and the server doesn't write anything in the client's
log even if I configured the log level as 3 in the smb.conf.
The server os level is 255, and the PDC works without any problem in the
LAN.

Someone could have some ideas about how to solve this problem?
I tried and tried but I didn't managed in solving it... :-(

Thank you very much!
Bye.