Several users on Reddit are reporting that American Express has already
updated their system to prevent the addition of non-Amex cards. This means
that the only way a non-Amex credit card load will work is if you use a card
already linked to your Serve account.
At this point I would highly suggest not deleting the current card linked to
your Serve account. While some people reported they were able to add a new
card in the past couple of days, quite a few have had issues and thus are
missing out on their last loads.
