I use external authentication as well as external enrolments and I disabled manual enrolments. When a user selects a course in which the user is not enrolled, s/he receives the message:Manual enrolments are currently not enabled. When the user selects continue he is redirected to the main moodle site, but receives a 405 error instead of listing the contents.
The page you are looking for cannot be displayed because an invalid method (HTTP verb) was used to attempt access.
This also happened on Version 1.5.2.