I just had the same issue trying to subscribe on SWTOR''s storefront. Kept giving me the "An error has occurred while trying to process your request. Please, try again later." message. When I came here to the ea.com support section I figured I should log in. My password was expired. I reset my password. I then went back to SWTOR website and was able to purchase the subscription.
When I had clicked on the "Subscribe Now" in the top left of the SWTOR page it said they had a card on file for me from 2016. Which was odd considering I had just updated my payment options with my current card when I previously attempted to subscribe a couple of minutes before. That was my first clue that they were retrieving info from somewhere else.
They are interfacing with your EA account, but not outwardly saying so. Please verify your EA account has an up to date password and also remove any old payment methods on the EA account. I hope that will fix some of the issues being experienced here.
PlaY-DeaD
Quando Omni Flunkus Moritati
When All Else Fails, Play Dead!