I think the "service" version is probably reading a different "Configuration file" to determine authorized clients (IP addresses) and/or users.

The error message seems to come from this code in RPC::PIServer:

  if (!($result = $self->AcceptUser($user, $password))) {
    $comm->Write($socket,
             [0, "User $user is not permitted to connect."]);
    return 0;
    }
[download]

"AcceptUser" authenticates the username against a user list, which I presume comes from a config file.

Sorry - no time to dig much deeper, but hopefully, this directs you toward solving this issue.

Remember that the SERVICE sees different environment variables (in particular PATH) than running from a cmd shell.