500 internal server error

Hi there. I am having trouble with a script which used to work but since moving to a new host I have changed a lot. Could someone please help me find where I am going wrong because the error I get doesn't give any kind of clue to what is wrong and I just can't figure it out. Here's the code:

<?php

//GET key: 1=not logged in, 2=user non-existant, 3=wrong password, 4=fields blank, 5=registered now login, 6=user already exists, 7=passwords don't match, 8=emails don't match, 9=email not sent

//This checks for invalid inputs
function checkOK($field) {
 if (strstr("\r",$field) || strstr("\n",$field)) {
   die("Invalid Input!");
 }
}

//random string generator
function randstr($md5 = "no", $length = "32", $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890') {
 //character list length
 $char_length = (strlen($chars) - 1);

 //start the string
 $string = $chars{rand(0, $char_length)};
 //generate string characters
 for ($i = 1; $i < $length; $i = strlen($string)) {
   //Generate a random character from the list
   $c = $chars{rand(0, $chars_length)};
//make sure same characters don't appear together
if ($c != $string{$i - 1}) {
  //add letter to string
  $string .= $c;
}
 }

 //optional md5 encryption
 if ($md5 == "md5") {
   $string = md5($string);
 }

 //return the string
 return $string;
}

//This code runs if the form has been submitted
if (isset($_POST['Submit'])) {

 //This makes sure they did not leave any fields blank
 if ((!$user) || (!$pass) || (!$pass2) || (!$email) || (!$email2)) {
   header("Location: register.php?reason=4");
 }

 //make field values into variables
 $user = $_POST['user'];
 checkOK($user);
 $pass = $_POST['pass'];
 checkOK($pass);
 $pass2 = $_POST['pass2'];
 checkOK($pass2);
 $email = $_POST['email'];
 checkOK($email);
 $email2 = $_POST['email2'];
 checkOK($email2);

 require('dbconfig.php');

 // checks if the username is in use
 if (!get_magic_quotes_gpc()) {
   $user = addslashes($user);
 }

 $check = mysql_query("SELECT username FROM users WHERE username = '$user'") or die(mysql_error());
 $check2 = mysql_num_rows($check);

 //if the name exists it gives an error
 if ($check2 = 0) {
   header("Location: register.php?reason=6");
   //check that the variable's length is ok
 } else if ((strlen($user) < 5) && (strlen($user) > 20)) {
   //if the username is not the correct length, redirect
   header("location: register.php?reason=9");
 }

 // this makes sure both passwords entered match
 if ($pass != $pass2) {
   header("Location: register.php?reason=7");
 }

 // here we encrypt the password and add slashes if needed
 $pass = md5($pass);
 if (!get_magic_quotes_gpc()) {
   $pass = addslashes($pass);
   $user = addslashes($user);
 }

 // checks that both emails match
 if ($email != $email2) {
   header("Location: register.php?reason=8");
 }

 //message subject
 $subject = "Social Space email verification";
 //unique random string for boundary string
 $random_hash = randstr("md5"); 
 //headers must be seperated with \r\n
 $headers = "From: no-reply@groupyournetworks.com\r\nReply-To: support@groupyournetworks.com";
 //add boundary string and mime-type application
 $headers .= "\r\nContent-Type: multipart/alternative; boundary=\"PHP-alt-".$random_hash."\""; 
 //message body

 //generate random code with md5 encryption to ensure uniquity (mostly)
 $code = randstr("md5");

 ob_start(); //Turn on output buffering
?>
--PHP-alt-<?php echo $random_hash; ?> 
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

Hi <?php echo $user; ?> and welcome to Social Space.
To complete your registration go to: http://www.groupyournetworks.com/authenticate.php?user=<?php echo $user; ?>&code=<?php echo $code; ?>

--PHP-alt-<?php echo $random_hash; ?> 
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

<h4>Hi <?php echo $user; ?></h4>
<p>To complete your registration click the following link: <a href="http://www.groupyournetworks.com.authenticate.php?user=<?php echo $user; ?>&code=<?php echo $code; ?>"><?php echo $code; ?></a></p>

--PHP-alt-<?php echo $random_hash; ?>--

 <?php
 //save message to $message and clear output buffer
 $message = ob_get_clean();

 if (mail($email, $subject, $message, $headers)) {
 } else {
   header("Location: register.php?reason=9");
 }

 // now we insert it into the database
 $insert = "INSERT INTO users (username, password, email, new, valid, code) VALUES ('$user', '$pass', '$email', '1', '0', '$code')";
 $insert2 = "INSERT INTO networks (ignore) VALUES ('ignore')";
 $add_member = mysql_query($insert);
 $add_member2 = mysql_query($insert2);

 mysql_close();

 header("Location: index.php?reason=5");
} else {
 die("The form must be submitted!");
}

?>

Thanks, Jim