PaterAlf: Yes, I just had contact with my bank and they think the same. Couldn't do anything right away, because they don't have the report by now (it can take some hours). But everything should be fine in a while (tomorrow the latest ) again.
I still wonder if they or GOG changed anything. It's not the first time that I made a number of small transactions in a row and I never had problems before.
I have the same issue. Contacted my bank. No issues on their side according to them. Waited for a few hours, it still doesn't work.
Than I bought a pinata using my paypal account. It uses my creditcard as I have no funds on paypal. That payment went without a problem.
Perhaps it takes some time to get reports through and perhaps it's just GOG.com that's blocked.