Hello Irith - I'm having the same issue. For years my students just clicked "Enroll in course (name)" and then paid via PayPal and they were automatically enrolled in the course.
Now all of a sudden, I show they've made a payment, but they don't get enrolled into the course. I've been going in and manually enrolling them.
Not sure what I'm supposed to tell my webhost, as their answer was "Go ask Moodel!"
I hate to bother you, but did you get this issue resolved, and if so, would you be so kind as to post the resolution here, so I may fix my problem as well?
Thanks so much in advance!