Hi Andre,
"Note: when checking the student participants on the test site , it indicates that the anonymised students have never loggged in, although the usage log indicates extensive activities by all students..." This is weird, there may be a bug. The last access value can be read from user_lastaccess database table and I don't see 'user_lastaccess' references in backup/ folder (this folder manages backup and restore)
Maybe this can highlight why there are no valid samples in the course: https://github.com/moodle/moodle/blob/master/lib/classes/analytics/target/course_dropout.php#L204. And this for invalid courses: https://github.com/moodle/moodle/blob/master/lib/classes/analytics/target/course_dropout.php#L126