iPayment / Authorize.net Issue - User authentication failed due to invalid authentication values

You can see such an error while doing different tasks with iPayment, for example, while adding a business partner credit card:

This error is generated by the gateway and can be solved by creating a new transaction key in the authorize.net website.

If that doesn't fix it, please check the following:

1. If you've entered wrong credentials in iPayment
2. If you're entering credentials to the live environment when using the test side or vice versa