Two possible things that might be an issue on Windows:
- If you have a browser extension installed like HTTPS Everywhere, it may be trying to load App Inventor over HTTPS which will break how App Inventor connects to the iOS companion.
- If you have a firewall configured on the computer, ensure that the browsers are allowed to make outgoing connections on TCP port 8001, which is the port used by the browser to talk to the companion app on the iOS device. If outgoing connections are being blocked, that would explain why all browsers fail to connect on Windows.