I have been testing LDAP course enrollment. I got a config that works and I was trying to delete all the existing classes and users again from scratch. Trying to delete users or courses post the following error:

DELETING CLASSES

Debug info: The ntext data type cannot be selected as DISTINCT because it is not comparable.
SELECT DISTINCT e.*
FROM mdl_enrol e
JOIN mdl_user_enrolments ue ON (ue.enrolid = e.id AND ue.userid = ?)
JOIN mdl_enrol pe ON (pe.courseid = e.customint1 AND pe.enrol <> 'meta' AND pe.courseid = ?)
LEFT JOIN mdl_user_enrolments pue ON (pue.enrolid = pe.id AND pue.userid = ue.userid)
WHERE pue.id IS NULL
[array (
0 => '11',
1 => '2789',
)]
Stack trace:
line 391 of \lib\dml\moodle_database.php: dml_read_exception thrown
line 247 of \lib\dml\mssql_native_moodle_database.php: call to moodle_database->query_end()
line 699 of \lib\dml\mssql_native_moodle_database.php: call to mssql_native_moodle_database->query_end()
line 728 of \lib\dml\mssql_native_moodle_database.php: call to mssql_native_moodle_database->get_recordset_sql()
line 159 of \enrol\meta\locallib.php: call to mssql_native_moodle_database->get_records_sql()
line ? of unknownfile: call to enrol_meta_handler::user_unenrolled()
line 296 of \lib\eventslib.php: call to call_user_func()
line 515 of \lib\eventslib.php: call to events_dispatch()
line 1174 of \lib\enrollib.php: call to events_trigger()
line 1380 of \lib\enrollib.php: call to enrol_plugin->unenrol_user()
line 727 of \lib\enrollib.php: call to enrol_plugin->delete_instance()
line 4178 of \lib\moodlelib.php: call to enrol_course_delete()
line 4131 of \lib\moodlelib.php: call to remove_course_contents()
line 3152 of \course\lib.php: call to delete_course()
line 149 of \course\index.php: call to category_delete_full()

DELETING USER

Debug info: The ntext data type cannot be selected as DISTINCT because it is not comparable.
SELECT DISTINCT e.*
FROM mdl_enrol e
JOIN mdl_user_enrolments ue ON (ue.enrolid = e.id AND ue.userid = ?)
JOIN mdl_enrol pe ON (pe.courseid = e.customint1 AND pe.enrol <> 'meta' AND pe.courseid = ?)
LEFT JOIN mdl_user_enrolments pue ON (pue.enrolid = pe.id AND pue.userid = ue.userid)
WHERE pue.id IS NULL
[array (
0 => '13',
1 => '2672',
)]
Stack trace:
line 391 of \lib\dml\moodle_database.php: dml_read_exception thrown
line 247 of \lib\dml\mssql_native_moodle_database.php: call to moodle_database->query_end()
line 699 of \lib\dml\mssql_native_moodle_database.php: call to mssql_native_moodle_database->query_end()
line 728 of \lib\dml\mssql_native_moodle_database.php: call to mssql_native_moodle_database->get_recordset_sql()
line 159 of \enrol\meta\locallib.php: call to mssql_native_moodle_database->get_records_sql()
line ? of unknownfile: call to enrol_meta_handler::user_unenrolled()
line 296 of \lib\eventslib.php: call to call_user_func()
line 515 of \lib\eventslib.php: call to events_dispatch()
line 1174 of \lib\enrollib.php: call to events_trigger()
line 1471 of \lib\enrollib.php: call to enrol_plugin->unenrol_user()
line 708 of \lib\enrollib.php: call to enrol_plugin->user_delete()
line 3597 of \lib\moodlelib.php: call to enrol_user_delete()
line 76 of \admin\user.php: call to delete_user()