.../wp-login.php?action=register returns 404

I’m not sure what caused this, but when visitors to my site hit the Registration button, they are taken to [domain]/wp-login.php?action=register which results in a 404.

I’m trying to track down if I have misconfigured some setting somewhere or something, or if a plug in update may have caused an issues. I haven’t made any significant changes to the site and it was working fine prior.

Any ideas on where to look?