Why are errors popping up?

Question

PHP

The problem is this, I made a registration form, created a database, posted it on an open server. Everything works, the data is entered into the database.
Then I uploaded it to the hosting, entered the data of my database and decided to check again. And then there are errors. I searched for solutions to problems, but did not find it.


Here is the code

<?php
                if($_COOKIE['user'] == ''):
            ?>
            <div class="row">
                <div class="col">
                    <h1>Форма регистрации</h1>
                    <form action="validation-form/check.php" method="POST">
                        <input type="text" class="form-control" name="login" id="login" placeholder="Введите логин"><br>
                        <input type="text" class="form-control" name="name" id="name" placeholder="Введите имя"><br>
                        <input type="password" class="form-control" name="pass" id="pass" placeholder="Введите пароль"><br>
                        <button class="btn btn-success" type="submit">Зарегистрировать</button>
                    </form>
                </div>
<?php else: ?>
     <p>Привет <?=$_COOKIE['user']?>. Чтобы выйти нажмите <a href="/exit.php">здесь</a>.</p>
<?php endif;?>

<?php
  $login = filter_var(trim($_POST['login']),
  FILTER_SANITIZE_STRING);
  $name = filter_var(trim($_POST['name']),
  FILTER_SANITIZE_STRING);
  $pass = filter_var(trim($_POST['pass']),
  FILTER_SANITIZE_STRING);

  if(mb_strlen($login) < 5 || mb_strlen($login) > 90 ) {
    echo "Недопустимая длина логина";
    exit();
  } else if(mb_strlen($name) < 3 || mb_strlen($name) > 50 ) {
    echo "Недопустимая длина имени";
    exit();
  } else if(mb_strlen($pass) < 5 || mb_strlen($pass) > 16 ) {
    echo "Недопустимая длина пароля (от 5 до 16 символов)";
    exit();
  }

  $pass = md5($pass."dfdfsdfsdfdsf");
  require "blocks/connect.php";

  $mysql->query("INSERT INTO `users` (`login`, `pass`, `name`) VALUES('$login', '$pass', '$name')");

  $mysql->close();

?>