For those of you who’ve enabled the Hyper-V role in Windows Server 2008. It has been determined that a certificate error can occur resulting in the inability to start or connect to virtual machines running on Windows Server 2008 or Microsoft Hyper-V Server 2008.  The Hyper-V virtualization host and the existing virtual machines will continue to operate without disruption.  A hotfix designed to prevent/resolve this error has been created and is available for download starting today. 

More info: KBArticle (967902)