Dogmaus: In my case, I tried to pay with Paypal and the process could not work, but the game was added to my library nonetheless. And that's what I expected, since it's a recurrent error on GOG when I "buy" stuff that's 0.0 €.
I see. I might try this at some other time in the future, but today for this single particular game, I don't think it's worth the hassle to deal with PayPal / virtual credit cards / re-configuring my payment details back & forth etc.
Dogmaus: Yes, the check out is frozen, but have you checked if the games has been added to your library anyway?
I did - nothing changed.
Anyway - thx for the help bro !! :))
Cavalary: Try adding a non-free game to cart, then select a (or a different) payment method, then remove the non-free game and try again.
Yes. Thx for the good suggestion, but my previous reply to @Dogmaus above (about avoiding hassles ...) still applies.
Cheers @Cavalary !!! :))