When users who have mailboxes in Microsoft Exchange Online look up the free/busy information of on-premises users, the free/busy requests fail. Instead of the expected availability information, users see hash marks (\\\\) in the Scheduling Assistant. This behavior may occur when the following conditions are true:
- You're running Exchange Server 2013 Cumulative Update 12 (CU12) in the on-premises environment.
- OAuth authentication is set up in your hybrid configuration.
A change was made in Exchange Server 2013 CU12 that prevents the free/busy request from being authorized correctly.
Article ID: 3193485 - Last Review: Mar 10, 2017 - Revision: 5
Microsoft Exchange Online, Microsoft Exchange Server 2013 Enterprise, Microsoft Exchange Server 2013 Standard Edition