If you receive the error message “root folder cannot be blank” in Outlook 365 for example, please follow these instructions to correct the issue:
Open your Outlook first, click on File and the Account Dialog should open. Choose the email account with the issue from the dropdown. Now click on the Account Settings Icon and click on Account Name and Sync Settings as you see here:
Last, add INBOX (all CAPITAL letters as shown) in the field Root folder path:
This should solve the error “root folder cannot be blank” in Outlook.