Hello,
Your sent a message via the "contact us" form and Nicolas answered you.
It looks like you did not see his reply ; please check your spam folder, etc.
Hi,
That error message indicates that you're using the AIM mode of the Authorize.net payment method.
Normally, after the ":" of that message, you're supposed to get the error message from Authorize.net.
Since you don't, and they tell you that they don't see the transactions attempts, I would suppose that the issue is coming from the URL you've configured in the settings of the payment method which is not correct and thus it's sending the transaction request to somewhere else which doesn't return anything valid.
Please check that the URL setting has the value
secure.authorize.net/gateway/transact.dll
which is the URL of the Authorize.net AIM payment gateway.
If that doesn't help, please provide a screenshot of the settings of your payment method.
Also, I would recommend doing a test with the "SIM". In that case, you'll be redirected to Authorize.net's payment page at the end of the checkout and you'll directly see the message from Authorize.net about what is wrong.
Best regards,
Nicolas
Regards,