Problem: When a user is given a shared mailbox, that mailbox should automatically appear in the user's Outlook client right under the user's folder in the left panel, within a couple of hours if not minutes. But some users never see this happening, and if I add this folder manually via Account settings > More Settings > Advanced > Add Mailboxes > name of mailbox, it will usually prompt for username/password again but will keep popping such prompt as if no valid credentials are entered even when the input is correct, or, the folder will show up but clicking it will only show further error about Exchange server problem. There are similar solutions about this but it never works on my case. They were talking about turning off / on cache, uncheck / check download shared folders, etc.
I searched and searched to no avail.
Solution: Upgrade Outlook 2013 to 2016.