PayPal IPN - Changing from Sandbox to Live stops working - php

Question

I am using the PayPal IPN System which works perfectly in sandbox mode. All the payments went through successfully and my site updated as required. So, being ready to go live, I change my sandbox links to live links:
$action = 'https://www.paypal.com/cgi-bin/webscr';
But now, whenever the payment form is posted to this address, it ends up at
https://www.paypal.com/webapps/hermes?token=...
which just shows a blank page. No error_log is created on my server.
Here is my form data before the link to PayPal is clicked.
<form name="frm_payment_method" action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_blank">
<input type="hidden" name="business" value="payments#domain.com">
<input type="hidden" name="notify_url" value="http://domain.com/ipn.php">
<input type="hidden" name="cancel_return" value="http://domain.com/cancelled.php">
<input type="hidden" name="return" value="http://domain.com/success.php">
<input type="hidden" name="rm" value="2">
<input type="hidden" name="lc" value="">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="currency_code" value="GBP">
<input type="hidden" name="page_style" value="paypal">
<input type="hidden" name="charset" value="utf-8">
<input type="hidden" name="item_name" value="1 Credit">
<input type="hidden" value="_xclick" name="cmd">
<input type="hidden" name="amount" value="1.50">
<p><button type="submit" name="submit" class="btn btn-primary">Proceed to PayPal</button>
Cancel</p></form>
Of course, replacing domain.com with my own correct domains.