This is not a pre-sale question but a support question and I see your subscription has expired and with that also access to support. If you wish to receive further support an active subscription will be required.
Nothing changed in RO Payments, assuming this is about that as you do not mention it. In the transactions list we do a check if the notification script is reachable and that is probably where the error comes from. Here as well you do not mention where in the backend you see this.
My guess is that either something changed on the server or there is just a temporary issue with the name resolution. That is something you can check.