I just performed a upgrade from version 2.9.2 to version 3.2.1. We use different wbservice to external site and since then some webservice no longer work, for example core_enrol_get_enrolled_users returns null when first returned correctly your subscribers to the course.
used:
http://[hostname]/webservice/rest/server.php?wstoken=[token]&wsfunction=core_enrol_get_enrolled_users&moodlewsrestformat=json&courseid=[idcourse]