SSL Invalid Grant

Hi

This morning clients report that the API connection to MYOB is returning:

{"error":"invalid_grant","error_description":"The provided authorization grant (e.g., authorization code, resource owner credentials) or refresh token is invalid, expired, revoked, does not match the redirection URI used in the authorization request, or was issued to another client. The authorization code has already been used.

The interface has worked for at least a year. How can I fix it?

THanks

Andrew