Error code and description
Error 521 - Enroll in MFA at site
Additional Authentication Enrollment Required (521): Please enroll in the new security authentication system that your bank's site has introduced.
What it means
Your bank requires that you enroll in a new authentication system.
How to fix it
Your bank has changed the login process. Visit your bank's site for additional information. Look for security notices from their customer service group. If there's nothing obvious, contact their customer service and ask if anything has changed that would affect "downloading information from a third party data aggregation service."