EWS Java API:会议请求已过期.日历无法更新

我正在使用Microsoft EWS Java API代表某些用户创建和接受约会.我这样做的方法是发送会议请求,然后在收件人的收件箱中设置收件箱观察者并接受传入的MeetingRequest项目:

import microsoft.exchange.webservices.data.core.service.item.MeetingRequest;

void accept(MeetingRequest request) {
  assert(!request.getIsOutOfDate());
  request.accept(false);
}

但是,当我尝试接受会议请求时出现此错误:

microsoft.exchange.webservices.data.core.exception.service.remote.ServiceResponseException: The meeting request is out of date. The calendar couldn't be updated.
    at microsoft.exchange.webservices.data.core.response.ServiceResponse.internalThrowIfNecessary(ServiceResponse.java:278)
    at microsoft.exchange.webservices.data.core.response.ServiceResponse.throwIfNecessary(ServiceResponse.java:267)
    at microsoft.exchange.webservices.data.core.request.MultiResponseServiceRequest.execute(MultiResponseServiceRequest.java:165)
    at microsoft.exchange.webservices.data.core.ExchangeService.internalCreateResponseObject(ExchangeService.java:270)
    at microsoft.exchange.webservices.data.core.service.response.ResponseObject.internalCreate(ResponseObject.java:120)
    at microsoft.exchange.webservices.data.core.service.response.CalendarResponseMessageBase.calendarSave(CalendarResponseMessageBase.java:98)
    at microsoft.exchange.webservices.data.core.service.item.MeetingRequest.internalAccept(MeetingRequest.java:218)
    at microsoft.exchange.webservices.data.core.service.item.MeetingRequest.accept(MeetingRequest.java:184)

这对应于此错误:ErrorMeetingRequestIsOutOfDate.我查看了MSDN的这个错误,但无法找到为什么这个错误可能发生在我身上.

我究竟做错了什么？