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The error code CAA50021 in Office 365 indicates that there is an issue with the user's credentials, specifically with the authentication token that was received after passing MFA. This error may occur due to various reasons such as an expired token, network connectivity issues, incorrect user credentials, or a misconfiguration in the Office 365 environment. To resolve the issue, the user can try to sign out and sign in again, clear their browser cache, review and update their account settings, or contact their IT support team for further assistance.