Authentication code not recognised via API for REA change

A client is using our system to submit a change of REA. When we try to submit the change, we get an error message that the authentication code is incorrect. When we check the authentication code elsewhere (ie the XML route) and for other filings for this company the response from CH is that it is valid.
So why is the change REA process rejecting it?

Given client confidentiality please contact me directly for specific information.

Hi Oli,

If you have not already done so, can we take this “off line” as you suggest, and contact me at xml@companieshouse.gov.uk

SDN