Hello,
Of course I have contacted SBB and Discover about this; this is only a question for the meantime.
On the SBB Mobile app for Android: I attempt to add a payment method to purchase my ticket, and get error 1403. I am using a Discover card, and Discover is a listed accepted credit card on the app. My Discover account does allow Swiss purchases.
I'm wanting to know if you have this problem, or especially a solution.