[PHP]Login per MD5 Funktioniert nicht?[PHP]

if(!defined('ACCESS')) header('Location: index.php');
	if($this->isLoggedIn()) {
		header('Location: index.php?p=account');
	}
	if(isset($this->request['login'])) {
		if(isset($this->request['Spielername'], $this->request['Password'])) {
			$username = $this->request['username'];
			$password = $this->request['password'];
			$password = md5($password);
			$stmt = $this->pdo->prepare('SELECT `Spielername`, `Password` FROM `users` WHERE `Spielername` = :username AND `password` = :password LIMIT 1');
			$stmt->execute(array(
				':username' => $username,
				':password' => $password
			));
			if($stmt->rowCount()) {
				$obj = $stmt->fetch(PDO::FETCH_OBJ);
				$_SESSION['USERNAME'] = $obj->Spielername;
				$_SESSION['PASSWORD'] = $obj->Password;
				$timestamp = time();
				$datum = date("d.m.Y - H:i", $timestamp);
				$ip = $_SERVER["REMOTE_ADDR"];  
				$host = gethostbyaddr($ip);
				$stmt = $this->pdo->prepare('UPDATE `users` SET `Lastlogin` = :datum  WHERE `Spielersername` = :username');
				$stmt->execute(array(
					':datum' => $datum,
					':username' => $username
				));
				$stmt = $this->pdo->prepare('SELECT `Admin`,`Reason`,`Date` FROM `bans` WHERE `Spielername` = :username LIMIT 1');
				$stmt->execute(array(
					':username' => $username
				));
				if($stmt->rowCount()) {
					$obj = $stmt->fetch(PDO::FETCH_OBJ);
					$error = "Dein Account wurde von $obj->Admin am $obj->Date gesperrt. Grund: $obj->Reason.";
				} else {
					$stmt = $this->pdo->prepare('INSERT INTO `lastlogins`(`Name`, `Date`, `Ip`, `Useragent`) VALUES (:username,:datum,:ip,:host)');
					$stmt->execute(array(
						':username' => $username,
						':datum' => $datum,
						':ip' => $ip,
						':host' => $host
					));
					header('Location: index.php?p=account');
				}
			} else {
				$error = 'Falsche Zugangsdaten.';
			}
		}
	}

public function isLoggedIn() {
			if(isset($_SESSION['USERNAME'], $_SESSION['PASSWORD'])) {
				$stmt = $this->pdo->prepare('SELECT NULL FROM `users` WHERE `Spielername` = :username AND `Password` = :password LIMIT 1');
				$stmt->execute(array(
					':username' => $_SESSION['USERNAME'],
					':password' => $_SESSION['PASSWORD']
				));
				if($stmt->rowCount()) {
					return true;
				}
			}
		}