The most probable reason for it is that the Remove CC info option is enabled for the Open status. If a realtime payment method is set up in Merchium, disable the Remove CC info option for the Open status (in the Administration > Order statuses section). When a customer clicks the Place order button, the order is registered at your Merchium as Open and after that the necessary information is passed over to a server of the payment method. But if the mentioned option is enabled, the credit card information is passed over as XXXX, the error will occur.
NOTE: If these instructions do not help, and an order still gets the Failed status when using a realtime payment method, please contact the Merchium support team.