So we fixed the issue, finally! What was going on? The API of our payment provider (Mollie) gave a handling error because certain fields were missing at the check-out (zip code, city and billing address). We are using Cartflows and Woocommerce Booster, and I noticed that Woocommerce booster “overrides” Cartflows. By fixing that, all the required fields on the check-out for Mollie are shown again and the issue is fixed.
