Azure部署网站后登录无法跳转至index.php，本地环境正常的问题求助

各位大佬好，我碰到一个奇怪的问题：本地调试的时候，登录成功后能正常跳转到index.php页面，但把网站部署到Microsoft Azure、数据库部署到freemysqlhosting.net（免费版）之后，登录验证通过了却没法跳转。

我已经尝试过一些解决方案，但都没解决问题。下面是我的login.php完整代码：

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<title>Variant'C - Your Covid Solutions</title>
<link rel="icon" href="../images/logo.ico" type="image/x-icon">
<link rel="stylesheet" href="style.css"/>
</head>
<body>
<?php
require('db.php');
session_start();

// When form submitted, check and create user session.
if (isset($_POST['username'])) {
    $username = stripslashes($_REQUEST['username']);    // removes backslashes
    $username = mysqli_real_escape_string($con, $username);
    $password = stripslashes($_REQUEST['password']);
    $password = mysqli_real_escape_string($con, $password);
    // Check user is exist in the database
    $query    = "SELECT * FROM `users` WHERE username='$username'
AND password='" . md5($password) . "'";
    $result = mysqli_query($con, $query) or die(mysql_error());
    $rows = mysqli_num_rows($result);
    if ($rows == 1) {
        $_SESSION['username'] = $username;
        // Redirect to user dashboard page
        header("Location: index.php");
    } else {
        echo "<div class='form'>
<h3>Incorrect Username/password.</h3><br/>
<p class='link'>Click here to <a href='login.php'>Login</a> again.</p>
</div>";
    }
} else {
?>
<form class="form" method="post" name="login">
<h1 class="login-title">Login</h1>
<input type="text" class="login-input" name="username" placeholder="Username" autofocus="true"/>
<input type="password" class="login-input" name="password" placeholder="Password"/>
<input type="submit" value="Login" name="submit" class="login-button"/>
<p class="link"><a href="registration.php">New Registration</a></p>
</form>
<?php
}
?>
</body>
</html>

想请教下大家，这到底是代码哪里出问题了，还是部署环境的问题呀？麻烦各位帮忙分析一下，提前谢谢大家啦😊

备注：内容来源于stack exchange，提问作者Fajar Lazuardi