Block definitions should be at the top level, not the included level

templates/forms/inputs/register-user_name.html.twig

@@ -1,4 +1,3 @@
-{% block input_register_user_name %}
 {% if 'user_name' not in form.fields.hidden %}
     {% if col_width %}<div class="{{col_width}}">{% endif %}
         <div class="form-group">
@@ -7,5 +6,4 @@
             <input type="text" id="input-register-username" name="user_name" placeholder="{{translate('USERNAME.CHOOSE')}}" class="form-control" autocomplete="off">
         </div>
     {% if col_width %}</div>{% endif %}
-{% endif %}
-{% endblock %}
+{% endif %}