At least in practice Vacuum Cleanner, console and gazebo failed to connect to server.
Do you have the latest RADI image? You can get it by running
docker pull jderobot/robotics-academy
Also, are you using the correct RADI launch instruction? There are some exercises that have slight differences. The one for Vacuum Cleaner is
docker run --rm -it -p 2303:2303 -p 1905:1905 -p 8765:8765 -p 6080:6080 -p 1108:1108 jderobot/robotics-academy python3.8 manager.py
It’s also recommended to start a new container for each exercise you use. If you are on the terminal where the container is running you can stop it by pressing ctrl+c on Linux.
If none of the previous work for you, try cleaning the cache on your browser, there might be an update that is not loading.