I just want to concur that I had what seems like the exact same situation, using Mautic v3.3.1. Having a wrong host entered into a configuration field completely hung the entire configuration part of Mautic.
Thanks to this post from EJL, I realized what it was. I had mistyped my host address for my monitored email. It was never able to connect, which I guess Mautic tries to do each time before it displays the configuration menu. I had to fix the error directly in the app/config/local.php. Very strange I could bring down a section of a web site with a seemingly passive typo. I hope this helps someone else!