https://www.theregister.co.uk/2017/08/03/microsoft_breaks_office_365_login/

Some Office 365 customers can't use Office, thanks to a login portal redesign.

For the "last few weeks", Microsoft has been quietly rolling out an opt-in redesign for login pages. The change for accessing Azure Active Directory, a cloud service that manages users for Office 365, also started Tuesday for coherency.