Came across this issue with an onprem installation of Exchange 2013.
Created a new room mailbox. Assign Full Access permission to a user, and when you attempted to expand the mailbox in Outlook it gave the error “Cannot expand folder”.
Manually adding the room mailbox to Outlook did not resolve the issue.
Fix was to open the mailbox in OWA first, go through the initial Language and Timezone selection. Then Outlook opened the mailbox fine.