Does the same issue occur also for a completely new set of documents - the same document type, customer number, sales area?
Are they using partners with the correct account groups?
Did anybody make changes to the partner determination settings in VOPAN?
Is this billing for end customer or it is inter-company billing?
If you also get errors at sales order creation for bill-to/payer even though master data is maintained correctly, check whether some user exit related to partner re-determination got disabled after the upgrade.