Don't know why, but trailing slash appears to be required

index.html

@@ -254,7 +254,7 @@
         })();
 
         converse.initialize({
-            bosh_service_url: 'https://conversejs.org/http-bind', // Please use this connection manager only for testing purposes
+            bosh_service_url: 'https://conversejs.org/http-bind/', // Please use this connection manager only for testing purposes
             i18n: locales['en'], // Refer to ./locale/locales.js to see which locales are supported
             keepalive: true,
             play_sounds: true,