Access to NT4 File Ressources denied from Windows 2003 System

Have a problem that it's not possible to access any ressources located on a
NT4 Server when using the Windows 2003 System account on a Windows 2003
System.

- access from Windows Explorer to \\nt4server\share from the Windows 2003
System works fine.
- access from the system account (at \\w2003server 12:00 /interactive
cmd.exe), then "dir \\nt4server\share" gives instantly "access denied". It's
not a permission issue, even access to a shared ressource located on a fat
drive (netlogon share) doesn't work.

Any advice? Thank you in advance
Franz