Use to_json

roles/matrix-etherpad/templates/settings.json.j2

@@ -73,8 +73,8 @@
   },
 {% if matrix_etherpad_admin_username != "" and matrix_etherpad_admin_password != "" %}
   "users": {
-    "{{ matrix_etherpad_admin_username }}": {
-      "password": "{{ matrix_etherpad_admin_password }}",
+    {{ matrix_etherpad_admin_username|to_json }}: {
+      "password": {{ matrix_etherpad_admin_password|to_json }},
       "is_admin": true
     }
   },