Quote Originally Posted by RixStix View Post
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.