Issue

When a calendar is shared with a free/busy level of permission, users cannot add the calendar to their calendar view.

Status

Microsoft is aware and has provided a workaround. A ticket has been opened with Microsoft about this issue and we will provide an update when available.

Workaround

To open a shared calendar, the user must grant permissions of Reviewer or higher. Or, you can view the user's Free/Busy information by creating a new meeting request and then adding that user in the Scheduling Assistant.