500 Internal Server Error occuring at Checkout. Why ?

Zen Cart has been happily running, no changes recently (since the last security update). I was concerned because I was not always receiving emails when an order was placed so did a couple of test orders and went through checkout via Paypal and cheque options. Once I click confirm order when going for payment method 'cheque' I receive the 500 Internal Server Error page, The server encountered an internal error or misconfiguration and was unable to complete your request. The order does go through and I receive emails as a customer and as store owner.

If anybody out there knows where I should start with correcting this error I would be so thankful !

Here is my server's error log :-

[Sun Nov 14 17:26:34 2010] [error] [client 95.148.238.220] malformed header from script. Bad header=No recipient addresses found i: index.php, referer: http://www.jezebelcharms.co.uk/index.php?main_page=checkout_confirmation
[Sun Nov 14 17:27:31 2010] [error] [client 95.148.238.220] malformed header from script. Bad header=No recipient addresses found i: index.php, referer: http://www.jezebelcharms.co.uk/index.php?main_page=checkout_confirmation
[Sun Nov 14 17:56:32 2010] [error] [client 64.125.222.16] File does not exist: /home/sites/jezebelcharms.co.uk/public_html/robots.txt
[Sun Nov 14 18:07:39 2010] [error] [client 66.211.170.66] malformed header from script. Bad header=No recipient addresses found i: ipn_main_handler.php
[Sun Nov 14 18:09:39 2010] [error] [client 195.93.21.72] malformed header from script. Bad header=No recipient addresses found i: index.php, referer: http://www.jezebelcharms.co.uk/index.php?main_page=checkout_confirmation

Maybe this will help someone figure it out !

Sounds like you've got something amuck with email configuration settings. Maybe bad email addresses?
Admin->Configuration->Email Options

Also, in looking at your site, I see that you've butchered all the metatags and broken all the SEO optimization buit-in to the original template. While that's unrelated to your checkout issue, it'll hurt you in search engine indexing.