It's obvious there are huge problems with the authorize.net AIM module. Now I know this is an open source free product, but some of us paid lots of money for a template that now doesn't process transactions that it did only a few months ago.
I've been in financial services tech support for years and if many people are getting the same error running the same code with the same set up on the authorize.net gateway, there's a problem with the module.
Someone has to fix this. You cannot run an e-commerce shop without an e-commerce payment processing application.

