Running the script to manage external database enrolments shows this message:
Notice: Undefined variable: extcourse in [path]\moodle\enrol\database\enrol.php on line 99
Notice: Undefined variable: extcourse in [path]\moodle\enrol\database\enrol.php on line 102
Notice: Undefined variable: extcourse in [path]\moodle\enrol\database\enrol.php on line 103
Notice: Undefined variable: extcourse in [path]\moodle\enrol\database\enrol.php on line 104

I found that the referred file uses a non-initialized variable $extcourse. Switching that variable to $coursefield, at the lines metioned above, solves the problem.