I create new instance of WP multi site 'subdomained' (the next one). Add WordPress MU Domain Mapping plugin. After complex configuration i receive information on new created mapped site:
The constant "COOKIE_DOMAIN" is defined (probably in wp-config.php). Please remove or comment out that define() line.
Of course wp-config.php is free of 'cookie_domain' definition. Server error_log is empty.
Domain Mapping: 0.5.2
DNS configuration: OK