Cumulative Update 8 for Exchange Server 2016 was released in December of 2017. Customers with Office 365 Hybrid Deployments will find that after upgrading, cloud users that are not archive-enabled will not be able to query free/busy information for on-premise mailboxes. This is a known issue that affects OAuth connectivity in Exchange 2016 with an expected fix forthcoming in Cumulative Update 9, first quarter of 2018.
Currently, the only workaround involves archive enabling all cloud mailboxes that need to perform free/busy lookups for on-premise mailboxes.
To verify whether you are affected by this issue, use the Test-OAuthConnectivity cmdlet. The cmdlet should generate the following response:
Exchange Response Details:
HTTP response message:
Exception:
System.Net.WebException: The remote server returned an error: (500) Internal Server Error.
at System.Net.HttpWebRequest.GetResponse()
at
Microsoft.Exchange.Monitoring.TestOAuthConnectivityHelper.SendExchangeOAuthRequest(ADUser user, String
orgDomain, Uri targetUri, String& diagnosticMessage, Boolean appOnly, Boolean useCachedToken, Boolean
reloadConfig)
Please contact Helient for more information.