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I’ve just upgrade from 1.6.4 to 2.1.3 without problems.
But after install the MSM, i had the same problem (Call to undefined function comments_opened_notification_title() )
And this cause the super admin could not log in, so i had to make a restore, and comment the line
$Q[] = “insert into exp_specialty_templates(site_id, template_name, data_title, template_data) values (’”.$this->db->escape_str($site_id).”’, ‘comments_opened_notification’, ‘“.addslashes(trim(comments_opened_notification_title())).”’, ‘“.addslashes(comments_opened_notification()).”’)”;
before to create a new site with MSM.