All dates result in “The appointments are fully booked.”

When using any employee that has been created from scratch, or employees from demo data that have been modified or even cloned, all dates result in “The appointments are fully booked.”

In the case of a demo-data employee, the original works fine, but clones get the error, despite being exact clones.

I even made a vanilla test installation (WP 5.1.15 as required for the current project) with nothing else running, and the error is unchanged.

What is the fix?

Looking forward to a resolution.

Thanks for your help.

(CodeCanyon Reference: https://codecanyon.net/item/fat-services-booking-automated-booking-and-online-scheduling/24214247/comments#)