Hi Andrew,
If you only try to update the preferred supplier field, do you get the same error message?
When I get unknown errors, I do try to make the file as simple as possible, to eliminate any other things that may be effecting it.
I would make a fresh copy of the template out of DTW, and populate only the item code and preferred supplier field and try that as first step.
Here is the SAP note that may assist as well (865191 - Data Transfer Workbench (DTW) Troubleshooting Guide) - http://service.sap.com/sap/support/notes/865191
Kind Regards,
Nick Lakasas