Hello, I think I found the cause of the problem!
As you can see on the screenshot. The theme itself has its own ( Less powerful ) access control, even if they are not enabled, it is causing conflict. If I disable URE, the theme’s access control returns the 404 fine. But if URE is enabled, it gets a blank page. I am no programmer, but I’ll contact the theme developers.
On the regular 2017 theme it works wonders.
I’ll let you know what happens.