Authorize.NET CIM keeps on spinning/redirecting after entering the card details.

This will happen with hosted payment methods if the customer logged in using a non-primary site domain and the sites Settings > Domains tab setting 'Redirect to Primary Domain' is not turned on (check marked).
If the site has at least one hosted integrated payment method enabled and you have at least one other domain listed besides your primary, you will need to turn on the 'Redirect to Primary Domain' setting to prevent this issue from happening.