For Authorize.Net customers, it's possible to get the following error window when processing transactions.
The error originates from Authorize.Net and is trying to inform you that you do not have the 'Transaction Details API' enabled.
This API is required for iPayment to work correctly, so enabling it is a must.
The way you enable it is by logging in to your Authorize.Net account, navigating to Settings and clicking the menu item shown below. Afterwards you'll be asked to provide your secret answer, and you can then click the 'Enabe' button.