first of all big THANK YOU for Ben for his help and for his great site (sorry, I didn't want to register so I couldn't thank for the previous answer)!
As I have learnt there are ticket vending machines at every RER station where you can buy youth weekend daypass and Mobilis too by paying with a bank card. Is my info correct?
I was in Paris recently and wanted to buy some tickets to the airport but trying several machines at Chatelet - Les Halles different type of bank cards were denied (embossed VISA EUR, non-embossed Visa HUF, embossed MasterCard HUF). It was somewhat odd because the machine didn't try to access the bank just immediately told us the card is denied. The cards were working at the hotel, in stores, etc.. Should these cards work? As an alternative can bank notes be used at the machine? Is there an office with personal at Parc des Expositions on Sundays to buy daypasses?
Thank you in advance again!
Paris Metro RER ticket machines generally accept any denomination credit card which is PIN protected, but only French bank (debit) cards (Carte Bleu). I say "generally" as this is from personal experience and I've been able to test with only a few different bank / credit cards which were French and foreign banks.
My guess is that the Paris city train authority (RATP) has a moderately low level of accepted risk when deciding to allow / deny automated purchases (i.e. ticket machines) using cards. Visa & the credit card processor provide fraud scores on every transaction that a merchant makes and the merchant can decide to approve or deny the transaction depending on that score. It could be that these transactions that were attempted happened to be above the fraud score that RATP prefers and simply denied them based on that alone and had nothing to do with sufficient funds on accounts or your own bank declining the purchases.
This unfortunately is very hard to determine with certainty as it would require investigation by the programmers behind the software of these ticket machines.
A couple alternatives to purchasing Paris train tickets when your bank or credit card is denied at the ticket vending machine is to purchase at the ticket window staffed by a human or use Euro cash / coin at ticket vending machines. The point of sale machines at ticket windows are more lenient on fraud as the human staff can ask for secondary identification which obviously reduces fraudulent use of credit/bank cards.
Pretty much every Metro / RER station has ticket offices/windows staffed by humans, everywhere in Paris. The only time the windows are closed is usually close to opening / closing times of the station. Also, with multiple entrances to a station, it's common that not all entrances have human staffed ticket windows. You'll have to wander about to find which entrance has the ticket window staffed by RATP personnel.
Thanks for the kind words about the site! Have a great trip.