During deeper checks (with the WP debugging feature temporarily enabled for error logging), we noticed a group of errors being logged in the site's "[login to view URL]" file upon trying to activate a license.
The issue is due to an underlying database issue where the "wp_usermeta" table structure (and possibly other table structures in the database) are invalid or corrupted. Essentially, as Elementor tries to update the relevant database values within this table, this underlying issue prevents this as this update entry does not have a valid autoincremented index number assigned to it; this, in turn, makes it invalid and returns the error message