The nice thing is, that I cannot logout from the system :-), and when I try to log in with different user, I have the same error message in debug.log:
[26-Oct-2013 09:57:30] PHP Fatal error: Cannot redeclare check_password_reset_key() (previously declared in /var/userdata/web/*****/website/www/wp-login.php:281) in /var/userdata/web/*****/website/www/wp-includes/user.php on line 1642
