One other thing in addition to Marks comments is that this connection
method has been desupported for security reasons. (I think I read this somewhere)

Anyways...
Try to connect as an actual user (i.e. system,internal or sys).
i.e.
connect sys/change_on_install as sysdba

Your method will work if you are using external authentification and your
current user has been created with external authentification.