Okay, in 1.3.5 there was no debugging support available in the module, so there's no "simple" way to diagnose exactly what's causing the problem.
1. If you intend to use AIM, you need to have an SSL certificate set up so you can accept secure transactions on your site. Otherwise if you collect CC numbers without SSL protection you subject you and your customers to fraud and theft problems.
But not having SSL isn't going to give that particular error.
2. In your Authorize.net account, there is a page which allows you to set a transaction "delimiter" and maybe even an "encapsulation" character. What do you have those set to presently? I think v1.3.5 expects the Delimiter to be a comma, and the encapsulation to be blank.
3. Do you have an "authorizenet" table in your database? If you do, then it's possible you might be able to get some debug response data from the records in it. If not, things get more complicated. Upgrading would be advisable.



