If you have set Yahoo mail account as your primary mailbox in Nokia Account, and the user ID is same as the one in your Nokia Mail account (e.g. firstname.lastname@example.org and email@example.com), the following error will appear: Nokia Mail powered by Yahoo! can't load. We were unable to open your mailbox. Try loading again. If you continue to have issues, please contact customer care.
To fix the issue, simply change your primary email to your other email address in Nokia Account.