Hi Jack,
Greetings. According to your description, may I confirm whether you are experiencing this problem when trying to log into Office 365 portal (https://www.office.com) or Office 365 desktop clients?
If the issue happens when trying to log into Office 365 portal, you may try to clear the browser's caches and try again with the browser's InPrivate mode to check the result.
However, if this issue happens with the Office 365 desktop client, I‘d like to recommend that you may Quick Repair Office applications to check if it helps. For your reference: Repair an Office application.
If you need further assistance, to help you better and further, please share more specific information of your problem with us and send us a screenshoot of the complete error message for further checking. Thanks for your understanding.
Best regards,
Jennifer