I'm having similar problems as you are describing here. Hopefully, your experience in overcoming them can point me in the right direction. I am using authroize.net AIM module 2.1 (in oscommerce 2.3.4). I am able to run test transaction through my testing account. However, when I try this in the live merchant account, I get the following error:
There has been an error processing your credit cardPlease try again and if problems persist, please try another payment method.
I get this error whether the "Verify SSL Certificate" option is set to true or false. Furthermore, when I test the API Server Connection on the admin side, I get the following error:
"Failed! Please review the Verify SSL Certificate settings and try again."
So this does appear to be a SSL verification problem. My hosting and SSL certificate are through GoDaddy, but it is a SHA-2 based certificate (see below) so it ought to work...right?
"Valid until Tue, 07 Jun 2016 21:32:38 UTC (expires in 11 months and 25 days)
Key RSA 2048 bits (e 65537)
Weak key (Debian) No
Issuer Go Daddy Secure Certificate Authority - G2
Signature algorithm SHA256withRSA"
Finally, I have tried to add the Root 2 Geo Trust Global CA to the authorize.net.crt file but this has not helped. In contacting both Authorize.net and GoDaddy multiple times, they both claim that everything is good on their ends (which may or may not be true). Any ideas?