Microsoft Knowledge Base Article 839499 contains the following summary:

This article discusses how to resolve the following two problem scenarios that may occur in Microsoft Windows Server 2003 or in Microsoft Windows 2000 Server:

  • Server message block (SMB) signing is disabled for the Workstation service on a domain controller, but SMB signing is required for the Server service on the same domain controller.
  • SMB signing is disabled for the Server service on a domain controller, but SMB signing is required for the Workstation service on the same domain controller.