Hi Martin,
Can you kindly check the following
1. There must be a Text Id.(ESIN) created for holding the external reference (delivered via a business set) can you check and see if the text's associated with the vendor created in ECC is referring to the SAP Sourcing External ID.
2. In the Vendor XML that is going back to SAP Sourcing, can you check the contents of the 'external_supplier_id', 'alt_external_id' and the external_id are correct.. (alt_external_id should be the same as the external_id)
Prasad