Şunları denediniz mi ?
https://support.plesk.com/hc/en-us/articles/213401789-Unable-to-log-in-to-Plesk-Unable-to-connect-to-database-mysql-connect-Permission-denied-var-lib-mysql-mysql-sock
https://talk.plesk.com/threads/after-the-update-error-db-query-failed-sqlstate-hy000-2002-connection-refused.358465/
