I've recently finnished my payment-setup for various payment methods but when I want to buy something from my store, I get an pop-up saying: "payment failed: unknown merchant account" and I get re-directed to the checkout page. My accounts for paying by credit card or other are all activated and verified. I've tried with other shipment-adresses but I have the same problem. What could be the problem?
Pretty sure this is an error on your payment providers end not Shopify. You state you get a pop-up with this message "payment failed: unknown merchant account" then redirected back to checkout. Which means you successfully selected a payment method and were redirected to that payment method page. Please check with your payment gateway provider Mollie.