I have a custom payment method that uses the
cc-form to take debit/credit card payments.
After the user has entered their card details, they are re-directed to my controller with a hidden form, then to the 3dsecure page.
After 3dsecure, they are re-directed to the same controller and various logic is checked.
After the logic is valid, they are re-directed to the checkout success page.
This was working with Magento 2.2, but in 2.3.0 this is not working.
After placing order, I am re-directed to 3dsecure as normal, but on returning from 3dsecure, I am re-directed to the home page with
"Invalid Form Key. Please refresh the page."