Impossible to upgrade from 1.9.x to 2.02
database and all tables using utf8_unicode_ci

Error message :
Debug info: Illegal mix of collations (utf8_unicode_ci,IMPLICIT) and (utf8_general_ci,IMPLICIT) for operation '='
SELECT DISTINCT c.id AS courseid, ra.enrol, c.timecreated, c.timemodified
FROM mdl_course c
JOIN mdl_context ctx ON (ctx.instanceid = c.id AND ctx.contextlevel = 50)
JOIN mdl_role_assignments ra ON (ra.contextid = ctx.id AND ra.enrol <> '')
LEFT JOIN mdl_enrol e ON (e.courseid = c.id AND e.enrol = ra.enrol)
WHERE c.id <> ? AND e.id IS NULL
[array (
0 => '1',
)]
Stack trace:

• line 391 of /lib/dml/moodle_database.php: dml_read_exception thrown
• line 749 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
• line 4084 of /lib/db/upgrade.php: call to mysqli_native_moodle_database->get_recordset_sql()
• line 1382 of /lib/upgradelib.php: call to xmldb_main_upgrade()
• line 273 of /admin/index.php: call to upgrade_core()