0x7 KDC_ERR_S_PRINCIPAL_UNKNOWN

An event log 3 about a Kerberos error that has the error code 0x7 KDC_ERR_S_PRINCIPAL_UNKNOWN for Server Name cifs/ will be logged when a share access is made against a server IP address and no server name. If this error is logged, the Windows client automatically tries to fail back to NTLM authentication for the user account. If this operation works, receive no error.

http://support.microsoft.com/kb/262177