I keep running into this problem, and going around trying to fix it, then eventually it does fix itself so Im assuming its a waiting game, but thought Id ask.
When I create a shared mailbox through the O365 portal, I add the user I want to be able to access it as a 'member' then I go over into Outlook and go:-
File / Account Settings / Account Settings / Click on the O365 account / More Settings / Advanced / Add Mailbox
The mailbox shows find, but when I try to expand I get:-
Cannot expand the folder.
Seems if I wait a few hours it eventually works, however it really is doing my head in at the moment, and wanted to know if it is just a case of waiting, or if there is another issue.