No meu caso acontecia porque activaram todos os plugins de autenticação das disciplinas.

“This seems to be a issue with course enrollment plugin. One of the enrolment plugins the Course -> Enrolments caused this issue for me. I just unchecked the plugin and it worked for me. But I havent figured out why that happens .”

http://moodle.org/mod/forum/discuss.php?d=122491&parent=649056