I installed 2.7.1 from the wordpress website twice and it both didn’t have:
‘define(‘SITE_ID_CURRENT_SITE’, 1);’
I’ve replaced it in the area you mentioned and I still get the same error.
If possible, could you provide the new WPMU 2.7.1 download that you downloaded or a correct version of the wp-config-sample.php file? The one I downloaded from here: https://wordpress.org/download/ didn’t have: ‘define(‘SITE_ID_CURRENT_SITE’, 1);’. in the wp-config-sample.php file.