Add this behind Reverse Proxy that serves SSL

[enoch85]

define('FORCE_SSL_ADMIN', true);
if ( isset( $_SERVER['HTTP_X_FORWARDED_PROTO'] ) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
        $_SERVER['HTTPS']='on';

[enoch85]

+this in Nginx

proxy_set_header X-Forwarded-Proto https;