When I go to customizer, it's showing a 404 instead of the site.

When I go to customizer, it's showing a 404 instead of the site. I've set up multisite using your tools. I'm on WP 4.5.1.

I've tried default themes. I've deactivated all but the domain mapping plugin. I have other non-MU installs on this same server that function as they should. Any ideas on what might be going wrong and how it can be addressed?

Thanks!