Same problem for me. RC2, RC4 ,and RC5 boards would act as NTP clients just fine to a 2.8.x system, but will not work properly once the NTP server MT was upgraded to RC4, even though Windows will still obtain the time from the upgraded RC4 MT just fine. Even though RC2, RC4, and RC5 boards will not "reach" an RC4 server(in my test bench setup), the same boards will synchronize to an external internet NTP server just fine.... What's up?
I haven't really had any probems reaching public NTP servers, with the exception of a few that wouldn't respond properly to the MT. When I would try a different public NTP server/address, it would then reach and synchronize... Maybe try different public servers?