Comments no longer exist

(Following up on an early query by the site owner, to which someone suggested we start a new thread.)

The existing comments on this site have disappeared, and no new comments can be entered. There have been ’empty’ email notifications, though I’m not sure if they’re all valid or some spam. I know that some legitimate people have entered a comment and it’s then failed to appear either on the website or in the Comments page behind the scenes.

Having gone through similar earlier comments about this issue, I have:
– deleted the old and re-uploaded fresh copies of the wp-admin and wp-includes files
– repaired the database
– deactivated and reactivated all the plugins
– deleted the one plugin we weren’t using
– reset the theme

Not sure what else to try! I haven’t checked them all, but am not having the same problem on other WordPress sites I manage – some of which use exactly the same theme and plugins.

The site with the problems: https://www.amgalant.com

Thank you for your help!