We are seeing the following error in our On-Premise Exchange server (2013) which is in a hybrid setup. We don't have any users complaining of the Free/Busy schedule not working and our testing shows it is working.
Process 18424: ProxyWebRequest FederatedCrossForest from S-1-5-21-4055847394-2181167018-4057280825-11054 to https://outlook.office365.com/EWS/Exchange.asmx failed. Caller SIDs: OAuth. The exception returned is Microsoft.Exchange.InfoWorker.Common.Availability.ProxyWebRequestProcessingException: Proxy web request failed. ---> System.Net.WebException: The request failed with HTTP status 404: Not Found.
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.EndInvoke(IAsyncResult asyncResult)
at Microsoft.Exchange.InfoWorker.Common.Availability.Proxy.Service.EndGetUserPhoto(IAsyncResult asyncResult)
at Microsoft.Exchange.InfoWorker.Common.UserPhotos.UserPhotoApplication.EndProxyWebRequest(ProxyWebRequest proxyWebRequest, QueryList queryList, IService service, IAsyncResult asyncResult)
at Microsoft.Exchange.InfoWorker.Common.Availability.ProxyWebRequest.EndInvoke(IAsyncResult asyncResult)
at Microsoft.Exchange.InfoWorker.Common.Availability.AsyncWebRequest.EndInvokeWithErrorHandling()
--- End of inner exception stack trace ---
. Name of the server where exception originated: MREDMEXCH12. Make sure that the Active Directory site/forest that contain the user's mailbox has at least one local Exchange 2010 server running the Availability service. Turn up logging for the Availability service and test basic network connectivity.
