May I know what is the operation you have done on the production order on the recent two days?
Does it only happens for this production order, do you meet the issue on other production orders.
you can try to refer a sales order in following way.
1.Open the production order.
2.and then you can see the General/Setup/and Reference session.
3. under the session tab, you can find the item reference:
then you can refer to one sales order in the field of:
reference type and reference number.