Clock skew is too large

The time difference between the appliance and your Active Directory Kerberos server is greater than three minutes. Update the time on your Kerberos server, or adjust the NTP Server setting (set at the bottom of the Configuration > System > Time Zone page) to synchronize the appliance to the same NTP source as your Kerberos server.

Microsoft Windows Server 2000 can be configured as an NTP source, see KB articles and Microsoft Windows Server 2003 is configured as an NTP source by default.