If a teacher disables Manual enrolment in Enrolment methods, none of the manually enrolled users can access the course anymore, even the teacher. They get the following message: 'You can not enrol yourself in this course.'
Th only solution that I found is to login as an administrator to enable the method again in that course.

Steps:
1) create a teacher account
2) create a course
3) manually enrol teacher as course's teacher
4) login as teacher
5) enter into the course
6) disable the manual enrolment in Course administration -> Users -> Enrolment methods -> Manual enrolments
7) the teacher can't enter in the course and get the message 'You can not enrol yourself in this course.'