Authorize.net SIM - Timeout issue

I upgraded a site today from 1.3.7 to 1.3.9d & set up the payment processor for Authorize.net SIM instead of AIM to be able to clear the PCI requirements.

When I placed a test order, I was taken to Authorize.net to enter CC info & it successfully captured & returned me to the site, but instead of the correct completed order page I was taken to a Timed out session where I had to log in again.

In checking w/ Authorize.net the order information is all there except the invoice #, but nothing is getting logged in ZC.

What settings am I missing? I've got the correct relay & receipt URLs set at Authorize.net

Thanks!
Christy