WordPress solved problem unable to login on sub-domain, after domain name change
During automated installation, the wordpress MySQL database is initialized with the old domain name. After update of domain or creation of sub domain, the database file is not updated. So after login, the login page(wordpress) will try to forward to the old domain name in the configuration file. This will cause the login to fail.
// ** MySQL settings - You can get this info from your web host ** //
/** The name of the database for WordPress */
/** MySQL database username */
/** MySQL database password */