We are having a problem with Exchange 2007 if we do a syncronize from Outlook 2007, whereby we get the error code above. I have done a search and found this : http://support.microsoft.com/default.aspx/kb/951576
but I have doublechecked this and its fine.
The url for the OAB looks fine, shows as https://smtp.domain.co.nz/oab
This client is using a purchased cert from godaddy.