Check_Process Errors

[sammirah]

I upgraded from 1.5.1 to 1.5.3 about 3 months ago, and everything seemed okay until a customer recently called to inform me that he received an error when making his payment. So I logged in to see if I could duplicate the issue and found that the cart is fine, the shipping is added without problems, tax is fine but when customer submits payment via credit card or check/money order it take them to a HTTP 500 Internal Server Error on https://atmospherebooks.com/webstore...eckout_process. When the page is refreshed it says: Array ( [0] => This transaction has already been marked as authorized (CartID : 47186) ), what can I do to fix this?