I have been dealing with SAME issue but I think I figured it out.
PayPal and Check/Money order purchases go through just fine. However credit card purchases kicked back to a "whoops session expired" screen. The CC charge would go through but the transaction would not register with ZenCart. This is the same situation as described in a post above.
To fix it:
1. I ensured the Authorizenet_AIM module was activated in ZenCart admin (payment module). I put the sort order of this as #1. I deactivated the regular Authorizenet module.
2. In your Authorize.net account, go to Account on the top toolbar, and then in the section called "Transaction Response Settings" select Relay Response. This screen asks you for the URL you want Authorize.net to return to after processing the transaction. I put my website's "Order Success" confirmation page URL.
3. To find the exact URL for that confirmation page, I processed a payment through check/money order and then when the confirmation page displayed, I copied the URL into the Authorize.net Response Relay request page.
I am not sure if you need to perform all those steps, or if just changing to the AIM would have worked without specifying the Relay Response URL.
I hope this helps. If someone sees a problem with what I have done, please let me know. All I know is that I have been working with IXwebhosting for 2 days trying to get this fixed and they ended up telling me they had no idea what to do. Tonight, I did what I stated about, and now my CC transactions work. :)




