Okay..I believe I have this resolved.
Choose. "Use Payment Processor" in the Authnet Config
When using Authorize.net, you MUST NOW use your API Login Number. This is the number that shows up AFTER YOU Login to Authorize.net, then select "Settings" Under Account.
THIS IS IT. THIS SOLVES IT. THIS IS THE ANSWER. HOW DO I KNOW? Because I tested it for another client having the same issue! If for some reason, this does not work for you, then you did something wrong and you need to check your settings!
Eric