It might be a browser thing. Have you tried another browser? I’m using Opera for my Webmin install and sometimes things hang and I have to hit stop and reload. My Firefox is so locked down I probably couldn’t use it for my Webmin/Virtualmin site.
Edit: Not only does that staff have to support multiple OS versions there are browsers to consider. I use Firefox, Chrome, Chromium and Opera on a daily basis. Occasionally, Brave.
fail2ban in our default configuration has nothing to do with local WebSockets connections (which is how the terminal works, Webmin is proxying for you, so if you’re not blocked from Webmin, you’re not blocked from the Terminal).
Look at the Webmin log for clues. Check the JavaScript console for errors.
Try a different browser. The Terminal requires pretty modern browser features, and also can be blocked by some extensions. Try an incognito/private window with no extensions enabled.