Authorize.net Error 13: SOLVED

This ought to be a simple thing, but I can't seem to find the problem.

I created a new sandbox account with authorize.net and put the login ID and transaction key into the plugin. Every time I try to submit a transaction I get an error (13) saying that the login ID or password is invalid or the account is inactive.

I have checked and re-checked the values. I tried the authorize.net login ID and the API login ID for the plugin's "Login ID" field. I cut and pasted the values so that I wasn't typing them in. I even went so far as to create a new sandbox account, tried the new values and the result is still the same. It doesn't matter if I use AIM or SIM, I get the same error.

I have used the sandbox before with another shopping cart and it worked as expected.

I'm kind of pulling my hair out about now.

Thoughts??

EDIT: I uninstalled the Authorize.net plugin and then reinstalled Hikashop to get it back. I re-enabled it and ... nothing different. I can't think of anything else to try!

EDIT: SOLVED! The problem is that when using Authorize.net's sandbox mode, you have to change the URL to test.authorize.net/gateway/transact.dll .