for oracle version 12.2.x users cannot login using case insensitive passwords, even though SEC_CASE_SENSITIVE_LOGON = FALSE if PASSWORD_VERSIONS of user is not 10g.
following sql should show the PASSWORD_VERSIONS for a user.
select USERNAME,ACCOUNT_STATUS,PASSWORD_VERSIONS from dba_users;
USERNAME ACCOUNT_STATUS PASSWORD_VERSIONS
--------------- -------------- -----------------
dummyuser OPEN 11G 12C
to make PASSWORD_VERSIONS compatible with 10g
add/modify line in sqlnet.ora of database to have SQLNET.ALLOWED_LOGON_VERSION_SERVER=8
restart database
change/expire password for existing user
new users created will also have same settings
after above steps PASSWORD_VERSIONS should be something like this
select USERNAME,ACCOUNT_STATUS,PASSWORD_VERSIONS from dba_users;
USERNAME ACCOUNT_STATUS PASSWORD_VERSIONS
--------------- -------------- -----------------
dummyuser OPEN 10G 11G 12C