Admin mode breaks with subdomains in latest WP

The issue could be with setting up cookie which may happen if you have different siteurl and homeurl. To give it a try, please edit your wp-config.php file and add these two:

define( 'WP_HOME', 'abc.example.com/'); 
define( 'WP_SITEURL', 'abc.example.com/'); 

Please replace abc.example.com with your actual subdomain. The thing to make sure is both URLs should match.

If above does not resolve, then one more thing you can try is setting your cookie domain to the subdomain

define( 'COOKIE_DOMAIN', 'abc.example.com/');