Actually the problem occurs with two different cards/card providers which used to work fine before iOS 17.6.
When I try from the Wallet app, in both cases the error message is (in French) "Impossible to add the card" with button "Add card later".
When I try from the providers apps, I get in one case "Card not added" with button "Configure later", and in the other case "Impossible to add card" with button "Configure later"--I also get an e-mail from the card provider saying "for security reasons".
Again, both these cards/providers caused no problem before iOS 17.6. This is very confusing (and a serious problem).
Thanks for your help.
Martin