For the availability check, during sales order creation (VA01), the system takes into account the requirements transferred to MRP. In some cases, an order cannot be confirmed, even if there is sufficient stock according to CO09.
If I change the order (VA02) and run the availability check again, now the system can confirm the order.
It seems that SAP performs a different type of availability check during VA01 as for VA02. Does anybody know where in SPRO I can customize the settings for this?