JSI Tip 8262. You cannot open file shares or Group Policy snap-ins when you disable SMB signing for the Workstation or Server service on a domain controller?

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.