I had been beating my head a little lately on two recent ActiveSync/OMA setups in Exchange 2003. We always follow the same path, and since most of our work deals with Exchange 2007/2010 lately, it has been a while since we have had to set this up for Exchange 2003.
All of the basics where checked (ssl settings, file permissions, iis permissions, FBA/exchange-oma vdir, etc.) but the Exchange Remote Connectivity Analyzer still failed the FolderSync step with the error in ExRCA:
An HTTP 403 forbidden response was received. The response appears to have come from Unknown. Body is: