Just to share what I did so that users facing the same problem can try the method that helped me out.

After literally countless contacts and help from Quickbooks, I was still not able to resubscribe using the bank card (from Natwest) that was already saved in my Quickbooks Account.

So I decided to try and not to use the resubscription page. Instead, I used another card to make the payment which went through. After that, I switched the payment method back to the original Natwest card that had problems, and this time it was fine.

So I think there is a bug with the re-subscription page itself.

But a pity that although the above solved my problem, after doing that I lost all my discounts and offers! And no matter what I say to QB support, I was told that nothing can be done to get my offers back.

(╯°□°）╯︵ ┻━┻