[Samba] "net rpc shutdown" fails on Windows 7 Pro

[André Weidemann]

Hi,
I am running Samba 3.4.0 under Ubuntu 9.10 and would like to shutdown a 
Windows 7 Pro machine using:
"net RPC SHUTDOWN -t 60 -f -I <ip-address> -U <username> -d1".

Invoking this command, unfortunately results in the following error:

cli_pipe_validate_current_pdu: RPC fault code DCERPC_FAULT_ACCESS_DENIED 
received from host <ip-address>!
Shutdown of remote machine failed!
rpc command function failed! (NT code 0x00000005)
initshutdown pipe failed, trying winreg pipe
Could not initialise pipe \winreg. Error was NT_STATUS_OBJECT_NAME_NOT_FOUND

My guess would be that a Windows Group-Policy is preventing the command 
from accessing the machine, but I don't know which one.

Can anyone point me into the right direction?

Regards.
  André