Can't change domain users password on Windows 7 Professional 64-bit joined to Windows 2003 server domain

We have succesfully joined pcs to Windows 2003 server domain but pc domain users of Win7 pc can't change password because Windows says a massage such that "Unable toupdate the password.The valueprovided for the newpassworddoes not meet thelength, complexity,orhistoryof the domain.", in every pc.

I have:
- done a "gpresult /V" on the pc and here there are the password policies:
...
        Criteri account
        ---------------
            Oggetto Criteri di gruppo: Criteri password
                Criterio:            MaximumPasswordAge
                Impostazione computer:  90

            Oggetto Criteri di gruppo: Criteri password
                Criterio:            MinimumPasswordAge
                Impostazione computer:  N/D

            Oggetto Criteri di gruppo: Default Domain Policy
                Criterio:            LockoutBadCount
                Impostazione computer:  N/D

            Oggetto Criteri di gruppo: Criteri password
                Criterio:            PasswordHistorySize
                Impostazione computer:  N/D

            Oggetto Criteri di gruppo: Criteri password
                Criterio:            MinimumPasswordLength
                Impostazione computer:  8
...

- on the pc, executed "secedit /configure /cfg %windir%\inf\defltbase.inf /db defltbase.sdb /verbose", advised in a forum, and then "gpupdate": computer and user policies updated succesfully;

- checked that in the registry there isn't the "HKEY_LOCAL_MACHINE\ SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Network" key: ok, there isn't;

- tried to enter in pc with a new domain user: windows let me change manually the password the first time (Ctrl-Alt-Canc - Change password), then, the second time, can't change password no more with the message above again ("Unable toupdate the password.The value...").

What can I do to find out what is wrong?

Thanks