This looks like a bug in Spring: spring-projects/spring-boot#27147
JS WebSocket class can't connect to the server and there is nothing to fix on our side.
The same happens with:
- console code ws = new WebSocket("ws://localhost: 35729");
- Live reload Chrome extension: it doesn't work with 2.5.2
Fixed in Spring Boot 2.4.9 & 2.5.3