Hello, Of course, I've contacted SBB and Discover about this; for the time being, this is only an inquiry.
On the Android version of the SBB Mobile app: When I try to add a payment method to buy a ticket, I get the problem 1403. I'm using a Discover card, and the app lists Discover as an acceptable credit card. My Discover card allows me to make transactions in Switzerland. I'm curious if you have encountered this issue and, more importantly, if you have found a solution.
Please try to make the purchase again on www.sbb.ch and possibly to register the method of payment in the login again. If this still does not work, please try to delete your browser history and cookies or use a different payment method.
Thank you for your understanding and have a nice day.