I have been using MasterCard for payments on GOG.com for over two months now without problems, but yesterday when I tried to pay for purchase I got the following error message:
We're sorry, but your payment attempt has been declined; this could have happened for two reasons:
1) some of the information entered in the form was incorrect; you can re-enter it 2 more times
2) the transaction has been automatically blocked by your bank and we recommend contacting them for assistance. Please note that some banks block all overseas transactions by default, until the customer authorizes them.
I have entered the correct information and I live in Europe, just a stone˙s throw away from Poland, so overseas transactions are not the issue here. I have contacted support a while ago, but so far, there has been no reply.
So I kindly ask fellow Gog members for suggestions and/or possible solutions.

Here are the things that I already had issues with:
Did you enter the card number without dashes and without spaces?
Did you enter the correct verification number?
And lastly, are you sure that your card hasn't expired?
Hope those are not too stupid questions, but you never know. I made those mistakes myself.
Anyway, if the card isn't expired, it might just be a bug or an issue with your bank, or with the payment provider that GOG uses.

Make sure you entered details correct. I know I've accidental entered wrong details before, despite being certain.
You might want to check any Ad/popup/script blockers if you have them running. Updates to these and the way you pay through GOG may cause it to fail if it previously worked.
It's also possible the transaction/banking system experienced some downtime when you were trying to pay.

There is no such thing as a stupid question.:)
I entered the card number without spaces or dashes, the verification code and expiration date were also filled out correctly.
Oh, and I can still pay with the credit card at other online stores, even the ones that are overseas.

How long ago did you contact Support? It may take 1 to 3 working days to get a response, but if you have not heard anything after this period, it might be an idea to send it again.

It`s been one working day since I sent the message.
I know I am an impatient fellow, however I just want a fast and definite response from GOG support so I know what steps I must take to remedy this.
Plus, I hate pestering the bank clerks for no good reason, seeing as how I can still pay at other sites.
And last but not least, I want to relax with good (old) games after a stressful week.

I just tried to buy a game and had the same trouble as Jager
I used a Mastercard, am also from Europe and got the same error message
Tomorrow I'll try again

Yep iam having the same problem with my mastercard. I frm Malaysia so the prob just not for europe. Already send a ticket & awaiting reply.

I got a message from support that they are looking into the problem, but since tomorrow is Saturday, I am fairly certain that the matter won`t be resolved over the weekend.
This seems to be a worldwide issue for certain Mastercard owners and I am pretty sure I know where the problem lies, however I am reluctant to broach the subject over the public forums.
(sigh) I guess that this will be a a non-retro gaming weekend for some of us.

Em, let`s just say that there probably exists a security issue with certain cards and the security measures (or lack thereof) on them.
Paypal is not an option for me right now, because making an account with them and linking my card would take 2-4 days. Probably a back-up plan for future accidents, but not this weekend.
As for gifting games, I have no friends on Gog and franky I want to pay for them myself. I mean, pay for the games, not friends.:)

I still cannot make the transaction.
I also contacted my bank and they said there are no issues on their end.