VSM CP error 500 in VCD connector

I'm in the process of performing a new installation of VSM CP for a client. The VCD connector is installed and testing successfully, the resources are all defined, but when I run a scan it imports about 20 resources successfully then fails with the error:

minorErrorCode="INVENTORY_OBJECT_NOT_FOUND" message="Could not find object with moref &quot;null&quot; and VC ID &quot;f0e40f3c-935b-4f73-b13f-17ddf886cf5e&quot; in inventory category &quot;VirtualMachine&quot;." majorErrorCode="500" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.vmware.com/vcloud/v1.5http://10.199.4.34/api/v1.5/schema/master.xsd"></Error>

   Status Code: 500
   Status Description: Internal Server Error
   Connector Type Name: Infra.Connector.vCloud.vCloudConnector
   Connector Display: vCloud Director Connector
   Query ID: Get absolute href
   Message: The remote server returned an error: (500) Internal Server Error.

This is a new one for me - has anyone come across it before?