Can’t access backend

I’ve been having some pretty serious server issues with my WordPress site. My hosting company initially told me it had to do with some bad requests, so I’ve been testing plugins and deactivating a few that use a lot of ajax requests incluidng Thrive Leads.
Dreamhost support documentation also recommends delete all relevant tables in my database so I did so. After cleaning things up, the site started running well, so I wanted to reactivate Thrive Leads. But with the missing database files, it wouldn’t work. Reinstalling it for some reason did not cause the plugin to reinstall the database tables. So I tried to install a backup database via Updraft Plus. This process worked fine, except now I cannot access my back end. I get the error message “Sorry, you are not allowed to access this page.”

My site also returns the error: A non-numeric value encountered in /home/walbur4/alphabetpublishingbooks.com/wp-includes/functions.php on line 64 on every frontend page. The theme appears to be rendering without any plugins at all.

Is there anything I can do besides a fresh install? I’m concerned that since restoring my database seems to have caused this error that my whole site is lost.

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