Quote Originally Posted by DrByte View Post
Those 302 responses are not part of a default ZC install. Do you have a URL-rewriter intercepting the URLs?
No URL rewriter installed.

Quote Originally Posted by DrByte View Post
Are the apache vhosts
apache vhosts => Sorry, I do not understand the question.

Quote Originally Posted by DrByte View Post
and all the .htaccess files identical between both servers?
No they are not identical.
test site has no .htaccess in root
prod site has .htaccess in root. Most lines are blocking bad IP and bots. some carryover from previous zencart recommendations.

Removed .htaccess and auth.net SIM completes transaction as expected. The line from the server log does contain 302

198.241.168.60 - - [14/Nov/2014:18:44:18 -0800] "POST /index.php?main_page=checkout_process&action=confirm&zenid=3ee5d47d6e3486a1e004e9 3ff23ee21a HTTP/1.1" 302 - "-" "-"
Thank you. Now to figure out the .htaccess but that should be an easy matter compared to getting here.