Query for multiple terms of a custom taxonomy on CPT gives 404

I have a custom post type with several custom taxonomies. I’ve got another plugin set up to filter the list of posts with checkboxes. This all works fine when I check a single term from each taxonomy. But when I select multiple terms from one taxonomy, the page 404s. It should just list all the posts which fit either of the terms (or even if there’s an and/or mistake, it should just show an empty list instead surely?). What’s going wrong here?

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

 

This site will teach you how to build a WordPress website for beginners. We will cover everything from installing WordPress to adding pages, posts, and images to your site. You will learn how to customize your site with themes and plugins, as well as how to market your site online.

Buy WordPress Transfer