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.
The Hybrid Configuration Wizard will configure your environment to use OAuth for free/busy sharing if your environment doesn't contain any Exchange servers earlier than Microsoft Exchange Server 2013 when you run the wizard.