I recently went through some PHP issues with my web host, so I wonder if they messed with files on the server.
Here’s a link to a screenshot of my public_html folder on the server: https://prnt.sc/116xwkz
The php version (5.6) your host is using is rather old. Perhaps your host disabled something. You could try updating it to php 7. You should be able to do this from your cPanel.
Some folders in your screen shot (client-response, admin, cart) should not be in the root of a Backlight 3 site.
The dates on those folders suggest that they may be leftovers from a previous version. I don’t know if they’d be causing problems or not.
those are the only Backlight folders and files that should show up in the root of the site
then there are usually host files/folders like php.ini, cgi-bin/ and any other folders you may have created for your own purposes (like for subdomains or site assets)
You could try updating the php version. 5.6 is pretty old now.
That looks like your site is trying to call code that is no longer there.
Can you try clearing the template cache by visiting Designer > Templates > Clear Template Cache in the Backlight admin?
If that doesn’t help, can you visit the Backlight Modules page and check that all of the modules are running the latest version?