So there I was, looking at the fancy XenCenter graphs when I noticed that two of the servers in the HA enable pool would not display the graphs. My first reaction was to panic and use some colorful language (just kidding).

The solution boils down into two:

1. The XenServer time/date is out of sync. Check to make sure NTPD is up and running. Best practice is to use an NTP proxy server on the local net for your XenServer hosts, this is especially important when you are running XenServer Essentials with HA enabled. Always trust but, verify. Make sure the time on each server host is accurate.
2. Close XenCenter, and then remove the logs under "%AppData%\Citrix\XenCenterMain.exe*\".  Restart XenCenter and test.

Enjoy!