When I try to create a new appointment using EWS Managed API, I get the following error.

The request failed schema validation: The element 'OptionalAttendees' in namespace 'http://schemas.microsoft.com/exchange/services/2006/types' has incomplete content. List of possible elements expected: 'Attendee' in namespace 'http://schemas.microsoft.com/exchange/services/2006/types'.

When I add an optional attendee it works but I do not need any optional attendee.
I tried
but no help. But removing it by using

public readonly static ExtendedPropertyDefinition PidLidOptionalAttendees = new ExtendedPropertyDefinition(DefaultExtendedPropertySet.Meeting, 0x00000007, MapiPropertyType.String);
solved my problem.