The Oracle error ORA-29024: Certificate validation failure indicates that the Oracle database was unable to validate the SSL/TLS certificate of the server it was attempting to connect to. This often occurs when using functions like UTL_HTTP.REQUEST or APEX_WEB_SERVICE.MAKE_REST_REQUEST to access HTTPS URLs.
As a troubleshooting step, try accessing a different endpoint, regardless of what it is. If you encounter the same issue, consider using a proxy.