Hi,
Normally on an SBS server this error is seen because the user is not a member of the "DOMAIN\
SBS Internet Users" group and therfore does nto have internet access. Also this can be caused by a general timeout situation, which is discusswed in the following KB
http://support.microsoft.com/kb/924705 If you have SP1 for Antigen then you already have this hotfix.
Please note that you should create the reg key described in the KB above and also increase the enginedownloadtimeout. The getfilehttptimeout is the timeout for the actual engine download, whereas the enginedownloadtimeout is the overall timeout setting for downloading, testing and bringing online of the engine.
I would recommend the following values
HKEY_LOCAL_MACHINE\SOFTWARE\Sybari Software\Antigen for
Exchange\EngineDownloadTimeout - 1500 (Decimal)
HKEY_LOCAL_MACHINE\SOFTWARE\Sybari Software\Antigen for Exchange\
GetFileHTTPTimeout - 90000 (Decimal)
I hope this helps
Alex