When configuring an Oracle DB as External Database, or when upgrading an Oracle DB that was previously configured as External Database in Automation Engine, the connection fails. Details show the error "ORA-28040: No matching authentication protocol".
It is likely that the authentication settings on the Oracle DB are restricted, and don't allow connections for all client version.
Automation Engine (14.# and 16.#) use the Oracle driver ojdbc14.jar, installed during Automation Engine installation. If this driver is not supported by your Oracle DB, you need to configure the External Database via DBMS type "Other" instead of "Oracle".
Follow the steps given below to configure an Oracle DB via JDBC