Only a single NTP server is accessible (warning msg)

At our remote office we have a Celerra NX4 and quite annoyingly it always has warning alerts on it saying that Only a single NTP server on the host machine is accessible and responding.  Here are the last 4 events in the log:

Feb 25, 2013 5:49:55 PM

Only a single NTP server on the host machine [10.1.1.2] is accessible and responding out of all [2] of the NTP servers.

Feb 25, 2013 6:48:51 AM

Only a single NTP server on the host machine [10.1.1.1] is accessible and responding out of all [2] of the NTP servers.

Feb 23, 2013 10:45:43 PM

Only a single NTP server on the host machine [10.1.1.2] is accessible and responding out of all [2] of the NTP servers.

Feb 23, 2013 6:44:17 AM

Only a single NTP server on the host machine [10.1.1.2] is accessible and responding out of all [2] of the NTP servers.

Yes we have 2 NTP servers defined, 10.1.1.1 and 10.1.1.2.  They are both Windows 2008 R2 domain controllers in our home office.  The site is linked via 30mbps Verizon Fiber (virtual private line). 

In our main office we have another Celerra NX4 with the same to NTP server addresses.  The home office Celerra never complains about NTP server accessibility.  Any ideas how to resolve this problem?  Can I (and should I) add a 3rd NTP server to this box?

From the CLI of this box, I can always ping and get great response to both 10.1.1.1 and 10.1.1.2.