Users cannot expand folders or see complete calendar data even though they have full permissions

Consider the following scenario:

• You're working in an Office 365 hybrid environment.
• You have a mailbox that is hosted on an on-premises server that is running Microsoft Exchange Server 2010.
• An Exchange Online user adds the mailbox to an Outlook profile as an additional mailbox.
• The Outlook Anywhere (RPC/HTTP) connection of the on-premises mailbox to the Outlook client is disabled.

In this scenario, some users cannot expand the folders of the on-premises mailbox, and they can see only free/busy data in the Outlook calendar even though they have Full Access or Editor/Reviewer permissions to this mailbox.

To fix this issue, add the on-premises mailbox as an additional account in the Outlook client. To do this, follow these steps:

1. Select File > Info > Account Settings.
2. On the E-mail tab, select New.
3. On the Auto Account Setup page, enter the mailbox name, the email address, and your account password. 
4. Select Next.

Note Make sure that you have Full Access permission to this mailbox.