- Credit card payments are configured for redirect (meaning the customer should get to the external payment page to enter the credit card details).
- The error message "Payment failed" appears in the checkout.
- In the log /var/log/ops.log the message "Direct Link Request/Response in Ingenico" appears at that time.
Maybe a third-party extension (e.g. "Offline Maintenance") is used to put the shop into a maintenance mode.
If you are logged in to the Magento admin panel and try to make a payment in the shop frontend at the same time, the sessions can get mixed up. The checkout then behaves as if the order has been placed in the admin panel. Then a Direct Link request is performed, because for backend orders, only Direct Link (inline) payments are possible.
If the maintenance mode is disabled, the problem should no longer occur. Either the third-party extension needs to be uninstalled, or this special behaviour needs to be considered every time when tests are performed.