basically, the issue is that you can't establish a http connection with the web server.
there are many factors will result this.
poor network connection, server side mod_security etc.
probably you can further identify this issue by check the logs at the web server side.
if network ok, you should see a log entry every time you refresh -- this tells at least the request has reached to the web server.
if it's easy to reproduce the issue, you can send us the WHM/root access to us and steps to reproduce, I can take a quick investigation.