This is what I would do:

a) Switch to AIM. It should work with no problem. It does require SSL though.

b) At least *try* it, to confirm that it does work fine, even though I suspect you're likely one of those who will dig his heels in and say no because you like the SIM idea better.

c) Then if SIM is still failing, I'd begin carefully checking every file to see if anything's different: http://www.zen-cart.com/wiki/index.p...Obscure_Issues

d) I'd also look at the SIM debug logs for evidence of what is and is not happening.

e) Since it *was* working fine, I'm inclined to suggest that the problem is with something you've changed, which should be revealed in the comparison in (c) above. You could also take a backup of what you have now, and then restore from a backup you have from before the problem started, and see whether that makes any difference.

f) Worst case I'd install another uncustomized copy of Zen Cart in another folder+database, as a test, and test SIM there as well, to see whether the problem may actually be with your SIM account or maybe with your hosting, etc.

g) I'd ask Authorize.net how far the transaction is really getting, and what exactly they see as the most likely specific reason for the problem.