Users can't log in sometimes
That can happen if the site is redirecting to a different domain or to a secure/non-secure protocol upon login. I.e. with/without www or to http/https. If so, you could set a base domain and cookie domain in settings.php, and/or enable www/non-www redirect in .htaccess.

It also seems there may be a problem with PHP 5.6.1.
https://www.drupal.org/node/2723927
https://www.drupal.org/node/2379019