Add this configuration to both host and clients configs.
<system.runtime.serialization>
<dataContractSerializer>
<declaredTypes>
<add type="System.Globalization.CultureInfo, mscorlib,Version=2.0.0.0, Culture = neutral, PublicKeyToken=b77a5c561934e089">
<knownType type="System.Globalization.GregorianCalendar,mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
</add>
</declaredTypes>
</dataContractSerializer>
CultureInfo contains a calendar field of the base class Calendar and it doesn't know about the specialized GregorianCalendar type, but we would expect that's taken care of by adding the knowntype attribute.
http://www.vistax64.com/indigo/65776-cultureinfo-serialization-issue-gregoriancalendar-wcf.html