Fresh install of 5.1.3 does not work

I have various WP installations on various subdomains. On one subdomain yesterday, I installed the SG Optimizer 5.1.3 plugin for the first time. (This WP install never had 5.1.2 or any prior version installed.)

Very oddly, on the SG Optimizer settings page, neither of the switch-toggle buttons are usable on the first tab of settings (Supercacher)– nor on any of the other tabs. They simply don’t move when clicked. More strangely, under Environment Optimization > Set your PHP Version, it says “YOU ARE CURRENTLY RUNNING ON PHP 0.” (I doubt that.) And the dropdown menu for PHP Management Method, although it does allow me to interact with it, does absolutely nothing when I click ‘Save’; it does not post back to the server at all.

I have tried this on Chrome and on Safari. I have tried un-installing 5.1.3 and then re-installing. My configuration in Siteground cPanel shows that it is turned on for this subdomain. This WP install does not use Wordfence, as noted in another threead’s issue. What am I doing wrong?

The page I need help with: [log in to see the link]