html - Strato HTTP Error 500 after uploading my profile.php

one text

I hope you can give me a lead how uploading my profile.php suddenly made it invalid to the server to proceed (?). I'm running my website online on Strato.de so I can check out if it works in a live environment. I renamed the file from reset.php to profile.php and suddenly became unable to be reached.

Test account: id: user | pw: test123

profile.php

<?php

session_start();
 
 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
    <title id="txt_white">Welcome</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.css">
    <link rel="stylesheet" href="/styles.css">
    <style type="text/css">
        body{ font: 14px sans-serif; text-align: center; }
    </style>
</head>
<body>
    <div>
            <ul class="navfont">
                <li><a href="http://saurusflex.de/">Home</a></li>
                <li><a href="http://saurusflex.de/login.php">Login</a></li>
            </ul>
      <div class="date">
            <?php echo "Last Update: " . date("d/m/Y  h:i:sa"); ?>
      </div>
    </div>
    <div class="page-header">
        <h1 id="txt_white">Hi, <b><?php echo htmlspecialchars($_SESSION["username"]); ?></h1></b>. <p>Edit account</p>
    </div>
    <p id="txt_white">
        <a href="reset.php" class="btn btn-warning">Reset Your Password</a>
        <a href="logout.php" class="btn btn-danger">Sign Out of Your Account</a>
    </p>
</body>
</html>

index.php

    <?php
    
    session_start();
     
    
    if(isset($_SESSION["loggedin"]) && $_SESSION["loggedin"] === true){
        header("location: home.php");
        exit;
    }
     
    require_once "config.php";
     
    
    $username = $password = "";
    $username_err = $password_err = "";
     
    if($_SERVER["REQUEST_METHOD"] == "POST"){
     
        if(empty(trim($_POST["username"]))){
            $username_err = "Please enter username.";
        } else{
            $username = trim($_POST["username"]);
        }
        
        if(empty(trim($_POST["password"]))){
            $password_err = "Please enter your password.";
        } else{
            $password = trim($_POST["password"]);
        }
        
        if(empty($username_err) && empty($password_err)){
            $sql = "SELECT id, username, password FROM users WHERE username = ?";
            
            if($stmt = mysqli_prepare($link, $sql)){
                mysqli_stmt_bind_param($stmt, "s", $param_username);
                
                $param_username = $username;
                
                if(mysqli_stmt_execute($stmt)){
                    mysqli_stmt_store_result($stmt);
    
                    if(mysqli_stmt_num_rows($stmt) == 1){                    
                        mysqli_stmt_bind_result($stmt, $id, $username, $hashed_password);
                        if(mysqli_stmt_fetch($stmt)){
                            if(password_verify($password, $hashed_password)){
                                session_start();
                                
                                $_SESSION["loggedin"] = true;
                                $_SESSION["id"] = $id;
                                $_SESSION["username"] = $username;                            
                                
                                header("location: home.php");
                            } else{
                                $password_err = "The password you entered was not valid.";
                            }
                        }
                    } else{
                        $username_err = "The username you entered was not valid.";
                    }
                } else{
                    echo "Oops! Something went wrong. Please try again later.";
                }
    
                mysqli_stmt_close($stmt);
            }
        }
    
        mysqli_close($link);
    }
    ?>
     
    <!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
        <title>Login</title>
        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.css">
        <link rel="stylesheet" href="styles.css"/>
        <style type="text/css">
            body{ font: 14px sans-serif; }
            .wrapper{ width: 350px; padding: 20px; }
        </style>
    </head>
    <body>
        <div>
                <ul class="navfont">
                    <li><a href="http://saurusflex.de/">Home</a></li>
                </ul>
          <div class="date">
                <?php echo "Last Update: " . date("d/m/Y  h:i:sa"); ?>
          </div>
        </div>
        <div class="wrapper">
          <h2 id="txt_white">Login</h2>
            <p id="txt_white">Please fill in your credentials to login.</p>
            <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
                <div id="txt_white" class="form-group <?php echo (!empty($username_err)) ? 'has-error' : ''; ?>">
                    <label>Username</label>
                    <input type="text" name="username" class="form-control" value="<?php echo $username; ?>">
                    <span class="help-block"><?php echo $username_err; ?></span>
                </div>    
                <div id="txt_white" class="form-group <?php echo (!empty($password_err)) ? 'has-error' : ''; ?>">
                    <label>Password</label>
                    <input type="password" name="password" class="form-control">
                    <span class="help-block"><?php echo $password_err; ?></span>
                </div>
                <div class="form-group">
                    <input type="submit" class="btn btn-primary" value="Login">
                </div>
            </form>
        </div>    
    </body>
    </html>

home.php

<?php session_start(); ?>

<?php 
if(!isset($_SESSION['id'])){
    die(header("location: 404.php"));
}
?>

<!DOCTYPE html>
<html lang="de-DE">

<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
<link rel="stylesheet" href="styles.css"/>
<link rel=stylesheet href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js"></script>
<title>Home</title>
</head>
<body>

<div>
<ul class="navfont">
<li><a href="http://saurusflex.de/">Home</a></li>
<div class="btn float-right">
        <a href="profile.php" class="btn btn-warning">Accountsettings</a>
        <a href="logout.php" class="btn btn-danger">Sign Out</a>
        </div>
</ul>
<div class="date">
<?php echo "Last Update: " . date("d/m/Y  h:i:sa"); ?>
</div>
</div>

<div class="hello">
<h1>Welcome, <?php echo htmlspecialchars($_SESSION["username"]); ?></h1>
</div>

<div class="container">
  <h2>Verzeichnis-Browser</h2>
  <p>Ein Abbild vom Verzeichnis-Browser //Upload-Funktion kommt noch</p>            
  <table class="table table-hover">
    <thead>
      <tr>
        <th>Filename</th>
        <th>Last Change</th>
        <th>Filesize</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td>test.docx</td>
        <td>01.12.2020 04:22</td>
        <td>23 KB</td>
      </tr>
      <tr>
        <td>teller.xml</td>
        <td>12.12.2020 14:11</td>
        <td>41 MB</td>
      </tr>
    </tbody>
  </table>
</div>


<footer class="footer">Copyright 2020</footer>

</body>

</html>

Source