E_WC_02 Error When Site is Using HTTPS

[gallagherwebsitedesign]

I'm getting an error "E_WC_21: User authentication failed due to invalid authentication values." - E_WC_21 is the error code for "A HTTPS connection is required." with Auth.net. But I'm serving the website over https and don't have any mix content issues (http content served over https on that page).

Here is how to recreate this issue:

1. Go to https://popstopsdev.com/popstops-garage-stop
2. Add the product to the cart
3. Proceed to card
4. Proceed to checkout
5. Try to checkout with a test card number from Auth.net, such as 4111111111111111
6. You'll get an error with code: E_WC_21