Installation keeps complaining that Unicode library (mbstring) should be disabled, even though it seems it already is
http://php.net/manual/en/mbstring.configuration.php#ini.mbstring.http-input

Drupal's code is checking it in core/lib/Drupal/Component/Utility/Unicode.php, but it won't even check if unless PHP version is &lt; 5.6.0.  If you are using PHP 5.5.x here's the check:

  if (ini_get('mbstring.http_output') != 'pass') {
    static::$status = static::STATUS_ERROR;
    return 'mbstring.http_output';
  } 


So I would double check the phpinfo via the Drupal interface at /admin/reports/status/php to make sure you are changing the config file that's used by PHP for Drupal.