This one seems different to all the previous questions about this error message.
I'm getting the following message "Invalid form key. please refresh the page." when trying to enter a discount code or when getting to the paypal review step.
Also had a couple of admin logins fail where it just appears to refresh the page, no error given.
I'm on magento 2.3.1.
This is where it's happening when trying to use a discount code: