I had something similar with a different corporate code recently whereby the reservation went through initially but when i had to make an adjustment I got the same error message. I didn't get to the point of waiting to find out whether the discount code would be honored and just got in touch with National to have them cancel the reservation as i couldn't do anything myself.
My best guess is that the code has been changed to something new and as such the 'old' code is no longer valid but until someone gets actual confirmation this is just a guess. Might be worth making a back up rez just in case...