Sometimes I'm receiving this message below during the customer login after being changed the cookie domain through the admin panel.
Do you know why and when this message shows up?
Invalid Form Key. Please refresh the page
If you receive the error message, in cases where the form_key should still be valid, it is in most cases an issue in the cookie setup. If Magento can not find the form_key from your cookie, the form_key validation will fail. Issues for this could be an invalid cookie-domain, cookie-lifetime, ...