Go to Administration > Security > Site security settings and ensure that the " Allow extended characters in usernames " is not selected Do a CURL REST call simulating a WS client. You need to replace the URL of your Moodle instance curl 'MOODLE_URL/lib/ajax/service.php' --data-binary ' [{"index":0,"methodname":"auth_email_get_signup_settings","args":{}}] ' | python -m "json.tool" Confirm that: you don't see among the returned information a field named "extendedusernamechars" Go to Administration > Security > Site security settings and ensure that the " Allow extended characters in usernames " is selected Repeat the previous CURL request curl 'MOODLE_URL/lib/ajax/service.php' --data-binary ' [{"index":0,"methodname":"auth_email_get_signup_settings","args":{}}] ' | python -m "json.tool" Confirm that: you see among the returned information a field named "extendedusernamechars" with value true or 1