Validator does not know password, so it can only see/validate login form. So login form has to be fixed to be xhtml compliant. Find code starting with <html> and ending with </html> and replace with following code inside password_protect.php
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>Please enter password to access this page</title>
<meta http-equiv="CACHE-CONTROL" content="NO-CACHE" />
<meta http-equiv="PRAGMA" content="NO-CACHE" />
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<style type="text/css">
input { border: 1px solid black; }
</style>
</head>
<body>
<div style="width:500px; margin-left:auto; margin-right:auto; text-align:center">
<form method="post" action="#">
<h3>Please enter password to access this page</h3>
<font color="red"><?php echo $error_msg; ?></font><br />
<?php if (USE_USERNAME) echo 'Login:<br /><input type="text" name="access_login" /><br />Password:<br />'; ?>
<input type="password" name="access_password" /><p></p><input type="submit" name="Submit" value="Submit" />
</form>
<br />
<a style="font-size:9px; color: #B0B0B0; font-family: Verdana, Arial;" href="http://www.zubrag.com/scripts/password-protect.php" title="Download Password Protector">Powered by Password Protect</a>
</div>
</body>
</html>