[Samba] WinXP user password not accepted - very strange !!!

[ROUMPEKAS XRHSTOSNEKTARIOS]

Dear friends,

I have an Ubuntu 8.04 server fully updated running Samba 3.0.28a. Currently the ubuntu machine acts as a PDC and all clients have joined the domain successfully (clients are all WinXP SP2).

The strange thing is only with one user where:
If the user logs in with his password all works well. If he restarts or shutdown his machine and tries to relogin the system tells him that his username/password is not correct.
If I reset his password @ Ubuntu with smbpasswd username and renter the exactly same password, the user logs in successfully!
This is the oddest thing. It appears as if this user's password is "canceled" every time despite the fact that the user has never changed his password during this period of time.

Does anyone have any ideas?

All WinXP SP2 clients have requiresignorseal = 0 and they have all joined the network in the exactly same way. All machines have machine-trust-accounts which have been created in the exactly same way. All users are working perfectly and all processes are running smoothly with not a single problem except the before mentioned.

Your help is greatly appreciated.

Chris