Good day. I also had a login problem after updating to version 5.7. The site has changed the standard admin login page, and configured a redirect for the standard login path.
I was able to regain access to the site by creating a new file from “wp-login.php” to new name for the new user 1***5.php.
After that I was able to enter my site.
Maybe someone will benefit from my experience ??
p/s
Sorry for my English, I use google translate ??