I have nothing to fix because I don't know what the problem is yet. Your error is coming from Authorize.net themselves. They're rejecting the XML. Have you tried contacting Authorize.net?
We have not touched Authorize.net payment gateway. There's no reason for it to spontaneously stop working other than issues outside of our control, which typically means Authorize.net changed something on their end.
You're welcome to PM (see link in signature) backend login credentials and will take a look, but you really need to contact Authorize.net and have them check their logs to see why it was rejected.