Outlook 2016 for macOS, Authentication Failure, O365

Intended Audience: People experiencing Authentication Failure with Outlook 2016 for macOS and O365. Symptoms: Outlook 2016 for macOS stopped being able to connect to O365 and check mail, no error displayed at this stage. Credentials are definitely correct, still able to access O365 mail via iOS app and browser. Deleting and re-adding the account within Outlook