PROBLEM
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:
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You're running Exchange Server 2013 Cumulative Update 12 (CU12) in the on-premises environment.
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OAuth authentication is set up in your hybrid configuration.
Note 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.
CAUSE
A change was made in Exchange Server 2013 CU12 that prevents the free/busy request from being authorized correctly.
WORKAROUND
To fix this issue, install Exchange Server 2013 Cumulative Update 15: KB3197044, Download, UMLang Packs
MORE INFORMATION
Still need help? Go to Office 365 for business on Microsoft Community or the Exchange TechNet Forums.
