I have to confess, I eventually did a search in Dreamweaver on the phrase:
"Your transaction could not be completed",
because it seems that the latest transactions are not even ARRIVING at authorize.net.
The Dreamweaver search function found that phrase in the checkout_failure.php file, as well as in eCartDesignObject.js and a few other locations.
I eventually decided to do this because authorize.net and other locations are extremely insistent that the fault is not with them.
So could there then be something going on within these files upon my own web server that is suddenly causing these failures?
I have looked in the confirm.php, but I of course do not see anything that could be causing this sudden failure. It of course makes no sense to me why there would suddenly be these failures when everything was working just fine before. But if transactions are now not even arriving at authorize.net, then perhaps you can think of something that might be causing this lack of information-transmission? I can't believe that someone hacked into those files and changed things, and recognize that that is clutching at straws, but I am lost for an explanation as to why those messages are coming up when I try to create a transaction.