After installing the Oracle 11 client version 11.2 and lower the access to a Oracle 12 database is denied.
First error is ORA-28040: No matching authentication protocol error which can fixed by adding the parameter
SQLNET.ALLOWED_LOGON_VERSION_SERVER = 10 in the sqlnet.org of the admin/network directory of the client. (Oracle doc )
But even after adding this the login to the database is denied and the error user access denied is returned. The reason behind is the field password_versiosn of the view dba_users. In order to change it to version 10 the sstem has to be changed to non case sensitive password and the user needs to be altered to update the password field. After the password_versions is showing 10 in addtion to the other ones.
Detailled instruction are here: Change intructions