ExchangeFreeBusyService error, "System.NullReferenceException: Object reference not set to an instance of an object." (4347933)

When a Notes user creates a meeting and tries to query free busy data for an Exchange (or Office 365) user, the query may fail with the following error:

2016-01-04 13:49:34,171 [19] INFO  AutodiscoverResultCache - user@domain.com found in cache. Returning https://outlook.office365.com/EWS/Exchange.asmx
2016-01-04 13:49:34,171 [19] INFO  EwsWorker - Sending EWS request to Exchange
2016-01-04 13:49:37,608 [19] INFO  EwsWorker - Receiving EWS response from Exchange 00:00:03.4425098
2016-01-04 13:49:37,624 [19] ERROR ExchangeFreeBusyConnector - GetFreeBusyInfo()
System.NullReferenceException: Object reference not set to an instance of an object.
   at Dell.CMX.API.Core.FreeBusyExchange.EwsWorker.CreateUserFreeBusyList(TimeWindow timeWindow, IList`1 emailAddressList, GetUserAvailabilityResults availabilityResults)
   at Dell.CMX.API.Core.FreeBusyExchange.EwsWorker.GetUserAvailability(Uri ewsUri, SmtpEmailList smtpEmailList, DateTime startTime, DateTime endTime)
   at Dell.CMX.API.Core.FreeBusyExchange.ExchangeFreeBusyConnector.GetFreeBusyInfo(SmtpEmailList smtpEmailList, DateTime requestStartDateTime, DateTime requestEndDateTime)