On December 19th Microsoft has announced thr Cumulative Update 8 (CU8) for Exchange server 2016.

With the CU8 Microsoft fixed the following problems:

4056329 Can’t access EWS from Outlook/OWA add-ins via makeEwsRequestAsync in Exchange Server 2016 and Exchange Server 2013
4054516 “Your request can’t” error when accessing an archive mailbox via OWA in Exchange Server 2016
4055953 The recipient scope setting doesn’t work for sibling domains in Exchange Server 2016
4055435 No MAPI network interface is found after you install Exchange Server 2016 CU7
4056609 Event ID 4999 and mailbox transport delivery service does not start after you install Exchange Server 2016 CU7
4045655 Description of the security update for Microsoft Exchange: December 12, 2017
4057248 Many Watson reports for StoragePermanentException in Exchange Server 2016


Issues with CU8

By installing the CU8 on a hybrid Exchange infrastructure it can point to free/busy issues.
This issue occurs when mailboxes are present on-premises and in the cloud. A free/busy query from Office 365 to on-premises will fail for cloud-based mailboxes if those mailboxes do not have an archive enabled. The reason of the problem occurs because, the OAuth connectivity fails.

All Office 365 users who have archive enabled, this problem don’t occurs.

How to fix the Issue?
The easiest workaround is, to enable archive for all cloud based mailboxes.

The problem should be corrected in Exchange 2016 CU9.

If the workaround to enable archive to all cloud based mailboxes is not a option for you, Microsoft has also announced a Interim Update, more information about the KB4058297 you can find here.
This Update is of course available at no charge by Microsoft