This issue is caused by the default setting for allowed logon version in the 12 database.
Note that the SQLNET.ALLOWED_LOGON_VERSION parameter has been deprecated in 12c.

That parameter has been replaced by these:

SQLNET.ALLOWED_LOGON_VERSION_SERVER=n
SQLNET.ALLOWED_LOGON_VERSION_CLIENT=n

Note that SQLNET.ALLOWED_LOGON_VERSION_CLIENT would be necessary on the server when the database is ‘acting’ as a client.  Such as the case of a database link.
There is  no need to restart either the listener or the database after this change.  See additional notes below.

See the following reference for more information about these settings.
https://docs.oracle.com/database/121/NETRF/sqlnet.htm#NETRF2010