Well I had an issue recently that might not be the same as you, but it sounds familiar. On some express checkout pages I was asked to add a new card and when I tried re-adding my existing card nothing happened at all. Not all merchants had that issue. I found a solution for me that is really just a bandaid solution, but I think Paypal needs to fix the problem with Express Checkout. I use Chrome and tried using an addon called "Quick JavaScript Switcher" (or you could just disable JavaScript in your browser temporarily I guess). When I got to that Paypal window I turned off JavaScript and it forced Paypal to go into what looks like a legacy mode, stating that some functions need JavaScript and it asked for a CAPTCHA then it went through to the confirmation page and everything worked as it should. Now only try this if you're comfortable with it, but I'm just saying it worked for me and I'm relieved. Again, it could be the country thing that others mentioned, but I thought I'd share my experience in case it helps. Let me know if it works for you.
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