No online payments for 2and a half weeks!!!

<p>I've been emailing and calling for 2 and a half weeks to get he below message resolved, and to date, no luc****! "This transaction has not been authenticated by your card issuer due to a system error. Please call ***or +27 11 ********** . Your purchase will automatically proceed in one moment, or click to proceed now. " It then returns to the merchant screen, with the following msg: "The ECommerce payment was rejected (X25)" This happens on more than 1 site when trying to complete online payments using my Mastercard Cheque card!!!!!!! </p>