Sometimes I run into this login loop and here is something that has worked:
” I went into PhpMyAdmin > wp_usermeta > and deleted the meta_value for session_token. “
It seems to be more frequent on multisite installations and on those it can take more steps. When the above fails, then adding the following lines to wp-config seems to work best:
define(‘ADMIN_COOKIE_PATH’, ‘/’);
define(‘COOKIE_DOMAIN’, ”);
define(‘COOKIEPATH’, ”);
define(‘SITECOOKIEPATH’, ”);
There is a goodStackOverflow thread here: https://wordpress.stackexchange.com/questions/259839/cookies-in-multisite-where-network-sites-have-their-own-domain-name