j'ai pourtant tout essayer mais rien !
Même ces requêtes ne changent rien :

ALTER TABLE mdl_user COLLATE utf8mb4_unicode_ci ;

ALTER TABLE mdl_user CONVERT TO character set utf8mb4 COLLATE utf8mb4_unicode_ci ;

Il faut peut être que je donne aussi : 
Stack trace:

• line 486 of /lib/dml/moodle_database.php: dml_read_exception thrown
• line 1245 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
• line 1571 of /lib/dml/moodle_database.php: call to mysqli_native_moodle_database->get_records_sql()
• line 1543 of /lib/dml/moodle_database.php: call to moodle_database->get_record_sql()
• line 4810 of /lib/moodlelib.php: call to moodle_database->get_record_select()
• line 4319 of /lib/moodlelib.php: call to get_complete_user_data()
• line 143 of /login/index.php: call to authenticate_user_login()