net.exe : difference between XP and NT4 edition ?

Hy folks,
I`m trying to migrate some clients form NT4 to XP PRO SP1, but there
are some "strange" things that happen when I try to "mount" a share
from the NT4 PDC : if I use the net.exe from Xp with something like [
net use u: \\srvname\user /p:no ] I`m prompted for login and password
[
>net use u: \\srvname\user /p:no
The password is invalid for \\srvname\user.
Enter the user name for 'srvname': username
Enter the password for srvname:
The command completed successfully.
>
]
while if I use the "old" NT4 version of net.exe (copied to the XP
machine and renamed to net2.exe) the share if mounted without any
problem and without an authentication request as it should be.
- the XP-Box is completely in the NT4 Domain as reported by "nltest
/sc_query"
- all the NT4 Versions (Server and Clients) are SP6
- all users are Domain Users regitered on the PDC

any Idea how to make NT4 PDC accept the XP net.exe command ?