ORA-28040: No matching authentication protocol exception after upgrade to Oracle Database 12c

This issue is caused by the default setting for allowed logon version in the 12 database.

Solution

Set these parameters at the lowest version level that is required in your environment.
Please add the below to sqlnet.ora file.

 SQLNET.ALLOWED_LOGON_VERSION_SERVER=10
 SQLNET.ALLOWED_LOGON_VERSION_CLIENT=10

There is no need to restart either the listener or the database after this change.