You said you used some "code" fixes to resolve this ? That error suggests that your authorize.net account hasn't been configured properly.
What are the authorize.net people telling you about the transactions you're testing with? They should be able to see them hitting their servers, and likely can see more detailed error messages etc.
If you turn on "Enable Database Storage" and peek at the contents of the authorizenet table via phpMyAdmin, what result messages do you see ?




