This may be way off as i don't know frontpage well at all...

The error for 12163 is ERROR_INTERNET_DISCONNECTED ( from wininet.h header file ). Stupid one, but IE doesn't think it's working in offline mode for some reason does it? 
 
Otherwise - it may be that you are using http 1.1 and the isp doesn't support it? You could test this through unchecking the http 1.1 setting at control panel / internet settings / advanced / http.1.1

other than that best i could suggest is take a netmon trace of the traffic going to the isp and see if anything jumps out at you - my guess would be something that the server returned that forced a client disconnect.