I think that there is a problem in the database plugin. I realized of it because in the PHP logs I found many lines like:
[Thu Aug 11 12:21:46 2005] [error] PHP Notice: Undefined variable: course in /u00/campus/web/virtual/enrol/database/enrol.php on line 72
I looked for it and I found that in the line 72, there´s a reference to $course->id but I can´t find any $course variable in that scope. I think there should be a reference to $courseid instead of $course->id.
I´m using Moodle 1.5.2
I´m not 100% sure that this is an error, so I prefer commenting it here and not to send it to the bugs database.