Relevant Product: Signature Manager Office 365 Edition
You have enabled Multi-factor Authentication for your Office 365 accounts and now you are getting issues with authentication in the Outlook Signature Update Agent.
Due to the authentication methods used by the agent, we are unable to authenticate an account using Multi-Factor authentication.
To resolve this issue, you will need to use an App Password and then authenticate the Outlook Signature Update Agent with this password when prompted.
Please see instructions here on how to create the app password