Auth.net SIM script timed out while trying to POST transaction results

[RixStix]

Those 302 responses are not part of a default ZC install. Do you have a URL-rewriter intercepting the URLs?

No URL rewriter installed.

Are the apache vhosts

apache vhosts => Sorry, I do not understand the question.

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.

[DrByte]

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. ...

Thank you. Now to figure out the .htaccess but that should be an easy matter compared to getting here.

I'd be very interested in knowing which statements in your .htaccess have caused this.

[RixStix]

I'd be very interested in knowing which statements in your .htaccess have caused this.

Found it in the section for blocking "bad bots"
Amazing what can ultimately cause so much grief and troubleshooting; especially for non-coders.
Thank you again for pointing me in various directions to isolate the problem.

    RewriteCond %{HTTP_USER_AGENT} ^$ [NC,OR]

[RixStix]

Sent a beverage or two of your choice.

Thank you again

[DrByte]

Cheers. Glad it's finally sorted out!