Rolling back to 2.12.1 solved the problem. If a voucher is applied, there is no error message anymore, but Klarna is missing from the payment options. Our other payment options besides Klarna are still available and working.
Before rolling back, the error message prevent the customer from confirming the cart even if Klarna was not selected as payment provider, making it impossible to order.
We can live with this, this fix is good enough for now.
I can send you the logs to help find the error, which logs do you need?
