This is a quick post to resolve ORA-28040 & ORA-03134, specially if you have recently upgraded to 12c.
ORA-28040: No matching authentication protocol error.
ORA-03134: Connections to this server version are no longer supported error.
Actually from 12c the default setting for allowed logon version is 11, that means by default you can only connect to 12c instance from a 11g or later client. So if you access a 12c instance from 10g client OR if there is a database link in 12c to a 10g database instance, you would face the ORA errors mentioned above.
Also SQLNET.ALLOWED_LOGON_VERSION parameter has been deprecated in 12c and instead we have SQLNET.ALLOWED_LOGON_VERSION_SERVER & SQLNET.ALLOWED_LOGON_VERSION_CLIENT parameters which serve the same purpose. So in order to resolve the issue please set these parameters to the lowest version level that is required in your env. Hence if you are using 10g client to connect to 12c instance please set following variables in the sqlnet.ora file:-
Hope this post help you in quickly resolving the ORA-28040 & ORA-03134 errors.