Microsoft Knowledge Base Article 811961 contains the following SYMPTOMS:

When you upgrade a Microsoft Windows NT 4.0-based primary domain controller (PDC) to Windows 2000 or Windows Server 2003 by using Winnt32.exe (including the /checkupgradeonly switch), the upgrade may not succeed. When this behavior occurs, the following error message is recorded in the System Compatibility report:

No quarantined trusted domains can exist during NT 4 PDC upgrade