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 !