In what years were the clients who demanded more problems with Certificate of deposit born?
more problems with Certificate refers to MAX(COUNT("Sub-product" = '(CD) Certificate of deposit'));
SELECT T1.year FROM client AS T1 INNER JOIN events AS T2 ON T1.client_id = T2.Client_ID WHERE T2.`Sub-product` = '(CD) Certificate of deposit' GROUP BY T1.year ORDER BY COUNT(T1.year) DESC LIMIT 1