Recent problem with Exchange Web Services (EWS) API for Exchange Online

Several months ago we developed a web app to display the conference room schedules on monitors at our office. It has worked like a charm until a couple of weeks ago. Nothing has changed as far as permissions or code, but now we get the following error in our C# code:

ErrorMessage "Proxy web request failed. , inner exception: System.Web.Services.Protocols.SoapException: Periods have duplicate IDs.\r\n at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)\r\n at System.Web.Services.Protocols.SoapHttpClientProtocol.EndInvoke(IAsyncResult asyncResult)\r\n at Microsoft.Exchange.InfoWorker.Common.Availability.Proxy.Service.EndGetUserAvailability(IAsyncResult asyncResult)\r\n at Microsoft.Exchange.InfoWorker.Common.Availability.FreeBusyApplication.EndProxyWebRequest(ProxyWebRequest proxyWebRequest, QueryList queryList, IService service, IAsyncResult asyncResult)\r\n at Microsoft.Exchange.InfoWorker.Common.Availability.ProxyWebRequest.EndInvoke(IAsyncResult asyncResult)\r\n at Microsoft.Exchange.InfoWorker.Common.Availability.AsyncWebRequest.EndInvokeWithErrorHandling()" string

The error is displayed after the following line is executed:

var availability = service.GetUserAvailability(aInfo, tw, AvailabilityData.FreeBusy);

and before the following line is executed:

foreach (Microsoft.Exchange.WebServices.Data.AttendeeAvailability ava in availability.AttendeesAvailability)

The autodiscover script executed without error, so we have an active connection to the server. Not sure what "Periods have duplicate IDs." means from the error message above. Has anyone seen this before?

Thanks