I am having the same exact problem. I am trying to use the Authorize.net SIM module with OSCommerce, but every time someone tries to pay, they are redirected back to /login.php?SESSIONID=biguglynumber and they are booted out of the shopping cart login. When they log back in, their items are still sitting there and they are never processed into OSCommerce.
I have always had "Check User Agent" turned off, so that doesn't appear to be the culprit of my problem.
Can anyone offer up a solution to this nasty problem?
I plan on getting the AIM stuff working as soon as we get SSL going, but I need an interim solution and I am afraid that this problem might happen with it too.
Many thanks in advance!