I believe the issue occurs because the same OrderCarrier is used after switching channels, and it has a reference to the Klarna order that was initiated on your first visit to the checkout page. If a reference exists it will try to re-use the same Klarna order.
Maybe you could try clearing the payments and deliveries of the order carrier to force a refresh?
In ControllerBase.Initialize there’s a check if the channel on the order carrier doesn’t match the channel on the current request, try adding this before the call to `cart.SetChannel(…)´.
cart.OrderCarrier?.Deliveries?.Clear();
cart.OrderCarrier?.PaymentInfo?.Clear();