Try to connect to 12c standby database , then get “ORA-01017” error.
$ sqlplus sys/password@TESTDBSTY as sysdba SQL*Plus: Release 126.96.36.199.0 Production on Tue Jan 15 18:07:54 2019 Copyright (c) 1982, 2014, Oracle. All rights reserved. ERROR: ORA-01017: invalid username/password; logon denied
After investigation, found the listener is running by 11R2 binary, instead of 12cR1 binary.
Restart the listener in 12cR1 ORACLE_HOME, then everything is fine.
Always run listener by using the highest version of all ORACLE_HOME.