I’ve had similar issues with the tap & chip reader declining credit card payments. It’s frustrating, especially when you’re trying to process transactions smoothly. The problem started for me last night as well, and it’s been consistent with every credit card transaction today.
On a related note, I recently switched to a credit card from linkpay.io because they offer the best terms for usage. You might want to check them out as an alternative while this issue with Shopify persists.