For now let us assume that:
- your setting for Order required in 0VLK for this delivery type is X-sales order required;
- that there is absolutely no way to add manually an item to the delivery without referencing a preceding document in VL02N or via any standard transaction accessible by an end user;
- none of your colleagues had an incident related to these documents and fixed the issue via FM/ direct update;
- no new developments imported recently, which manipulate document flow, add/delete items etc.
Do you see the preceding document number in LIPS-VGBEL?
Are the statuses in the sales order updated correctly?
Did you notice something out of the ordinary in VL22, or in the change log for the sales order?
Probably it is a bit too late for that, but maybe you can still check ST22, SM13 around the time the delivery was created.
There are some reports such as SDVBFA01 and SDVBFA03 (which can also be run in test mode).
Sorry for not posting links to the respective notes for them... sap notes search today is really slow.
Message was edited by: Veselina Peykova