I just tried to upgrade my site from PHP 7.0.17 to PHP 7.1.3, but when I do this, Hustle stops working. The email popup I use is not displayed, and I cannot edit any of the popups settings. When I go to the Email Opt-in screen, all I see is the EDIT YOUR OPT-IN header, and none of the settings are available below this. I reverted my site back to PHP 7.0.17 and the popup is displayed again, and I can see and edit settings.
I don't have a staging server, so I can't provide you access to take a look for yourself, but if you have the same setup, I'm sure you'll see the same issue. Note that I'm also using nginx, not apache.