Not Retruned back to checkout success using ANZ payment gateway module and Simple SEO

Hi There,

All of a suddent some of our my clients customer haven't been redirected to checkout success page. I am using ANZ payment gateway module. After customer clicked pay at the gateway it says "Transaction approved" but not redirecting back to the site. It was showing runtime exception as given in the image.

When contacting the bank, they said "This error is occuring because the returnurl (the page that migs returns the receipt data to) is incorrectly handling the data and resending another transaction request.". My return url is CHECKOUT_PROCESS but i cant find anything suspecious ter. I am using Simple SEO URL as well but I put exceptions to all checkout pages. Could any one please help me. Thanks:oops: