Well ... I'm having a hard time guessing exactly what the error says ... and thus I can't tell you what specifically might be wrong.
I would figure that if the things' truly broken it would be very quick and easy for you to go and simulate the problem and quote the exact actual error message if you really wanted support. It's very possible you have a setting set incorrectly somewhere.
Upgrading to the latest officially supported software version will probably resolve the matter (unless it's just a settings issue), since the authorize.net modules received a significant overhaul in v1.3.8. I just figured getting the real error message might be a little easier and faster than doing an upgrade.



