I have been getting a similar error message when trying to purchase an upgrade. I chose 'change seats' on an existing reservation. After cycling through the flights, I am offered an upgrade to first class by segment. I choose to upgrade on a longish segment. I select a method of payment and reach an 'agree to stipulations' check box. After that, I get the error message. I tried over about a week with the same result. I checked my credit card and found a charge pending for each time it bombed off.