Cannot add/edit user

I’m trying to add a new user to my WordPress site (worldnewseire.com) and when I’ve entered all the details and click “add user”, it redirects me to a 404 page on the main site.

I’ve tried using the Twenty themes and disabling all plugins. The same thing still happens.

I’ve also tried to see if I could circumvent that by enabling registration, but if I try to change the user’s role or add a name, I get the same 404 page.