Hi Nick!
The short answer: Unfortunately, there is no fast and workable solution for now. But the developer of the Divi theme can fix that issue.
The thing is, some themes try to adjust a page layout depending on what URL has been requested. That’s fine and normal. In the case of wp-admin a theme see it as an admin request. Why does a theme detects admin requests for a 404 page and tries to handle it in some wrong way? Everything depends on what was on the developer mind when she/he coded the theme. There is nothing wrong with detecting what part of WP is loaded, but if a developer of a theme implemented it in some not neat way that can be a cause of some issues, and as a result the theme are unable to render the 404 page for admin requests.
To be exact, a theme should not detect admin requests and render pages or load some stuff in a different way at all. Just because there is no reason for it.
The most common issue is that a developer doesn’t know how to load admin CSS styles and admin JS scripts correctly and we see a blank page or a non styled page. The only correct way is to use an admin_enqueue_scripts hook. Some developers just load admin stuff by checking for an admin URL. Some, not so less educated, use is_admin(). Both ways are wrong.
P.S. @avpman If you’d like to assist me with supporting the plugin and get some perks from it, drop me a message here with your contact email: https://wpcerber.com/support/