I followed the process here:
At the end there is an Important note about an exchange online user getting this error (which I am getting):
Cannot expand the folder. The set of folders cannot be opened.
Network problems are preventing connection to Microsoft Exchange.
I moved a mailbox from On-Premise to the cloud, which left a 'Remote User Mailbox' object behind, so the user has an exchange object On-Premise. I tried deleting that object and created a MailUser object instead, but I still had the same issue (I since changed it back).
Any troubleshooting suggestions?
In order to access to Public Folder in Exchange on premise. The mailbox Office 365 must be linked to an on-premises user account.
Do you have the mailbox online linked to an on premise account?